feat(serve): land the spawn-and-return plumbing + exit/status mechanics (w2-spawn-return)

- src/serve.rs (new): the ADR-006 decision-4 mechanics in code —
  spawn_serving_path (the one choke point; nothing awaited inline)
  and report_exit_status_then_close (exit-status before close via
  the cloned Session::handle()), #[doc(hidden)] contract-test
  re-exports; a unit row pins spawn-returns-while-pending.
- src/handler.rs: the four session-row seam stubs became spawned
  serving paths (failure-then-close unchanged; stubs are plain fns —
  an async stub holding &Session across the spawn is non-Send); open-
  level declines and the tunnel seam stub stay inline by class, per
  the new module-doc section.
- tests/spawn_return.rs (new): two end-to-end wire rows — the client
  observes exit-status(7) strictly before close and after the handler
  returned; the handler-returns-fast row routes follow-up traffic
  while the serving path is still gated (POC-3 deadlock inversion).
- tasks/implementation/w2-spawn-return.md — completed with notes.

Verification (all green): cargo test; per-feature dispatch matrices
(--features git/session/tunnel/sftp, --all-features); cargo clippy
--all-targets/-all-features with -D warnings; cargo fmt --check;
cargo doc --no-deps (no warnings); taskgraph validate.

src/handler.rs

@@ -69,7 +69,30 @@
 //! the store-and-take handoff (`channels.remove(&id)` → `into_stream`
 //! in the spawned serving task) is wave-3 mapping material; the
 //! routing decisions around it are already this wave's, because the
-//! routing stubs decline at the seam.
+//! routing stubs spawn their decline at the seam.
+//!
+//! ## The spawn-and-return structural constraint (ADR-006 decision 4)
+//!
+//! russh at this pin runs handler callbacks **inline inside the
+//! session's packet loop** — a blocking handler suspends all packet
+//! routing and deadlocks against the peer's follow-up bytes (POC-3
+//! proved by construction with receive-pack). Every serving path is
+//! therefore spawned through [`crate::serve::spawn_serving_path`] and
+//! the handler returns fast: the session-row seam stubs clone
+//! `session.handle()`, spawn their decline through the clone
+//! ([`crate::serve`]), and are gone from the hook's stack — no hook
+//! awaits a serving path inline. The decline keeps its wire shape
+//! (failure then close), riding the spawn. Exit/status reporting from
+//! spawned tasks rides the same cloned handle
+//! ([`crate::serve::report_exit_status_then_close`] — the mechanics;
+//! the derivation rules are wave-3 `mapping.md` material), pinned
+//! end-to-end by the mechanics rows in `tests/spawn_return.rs`.
+//!
+//! The open-level declines (unfeatured/never-feature rows, the tunnel
+//! seam stub) stay inline by class: they answer the open *before* any
+//! channel is owned — nothing is served yet, so nothing spawns. Wave
+//! 3's accept-only-after-fabric-open flow is where the tunnel row
+//! gains its spawned serving path, per the same constraint.
 //!
 //! Request-level declines: `subsystem_request` declines every name —
 //! `sftp` is the OQ-13 placeholder (the feature exists, the route
@@ -101,7 +124,9 @@
 //!   usable (the POC-1 §5 session-survives-declines posture); a
 //!   *serving-path* decline (the wave-3 seam stubs) answers failure
 //!   and closes the channel — the vanilla-clean shape for a row that
-//!   will spawn its serving path next wave.
+//!   owns the channel. The serving-path decline now rides the spawned
+//!   serving task (ADR-006 decision 4, §The spawn-and-return
+//!   structural constraint above); the wire shape is unchanged.
 //!
 //! ## Test surface
 //!
@@ -118,13 +143,16 @@ use std::sync::Arc;
 
 use alkcall::core::auth::Identity;
 use russh::keys::Algorithm;
-use russh::server::{Auth, ChannelOpenHandle, Msg, PublicKeyOffer, Session, VerifiedPublicKey};
+use russh::server::{
+    Auth, ChannelOpenHandle, Handle, Msg, PublicKeyOffer, Session, VerifiedPublicKey,
+};
 use russh::{Channel, ChannelId, Pty};
 
 use crate::config::ServeConfig;
 use crate::dispatch;
 use crate::error::AdapterError;
 use crate::fingerprint::fingerprint;
+use crate::serve;
 
 /// The per-connection handler (one per mounted stream): russh constructs
 /// nothing here — [`crate::SshAdapter::handle`] builds one per accepted
@@ -403,14 +431,14 @@ impl russh::server::Handler for SessionHandler {
         match dispatch::classify_exec(command) {
             dispatch::ExecRoute::Git => {
                 if self.serves_git_row() {
-                    git_row_stub(channel, session).await;
+                    git_row_stub(channel, session);
                 } else {
                     decline_unfeatured_request(channel, session).await;
                 }
             }
             dispatch::ExecRoute::Pipe => {
                 if self.serves_session_rows() {
-                    pipe_row_stub(channel, session).await;
+                    pipe_row_stub(channel, session);
                 } else {
                     decline_unfeatured_request(channel, session).await;
                 }
@@ -433,7 +461,7 @@ impl russh::server::Handler for SessionHandler {
         session: &mut Session,
     ) -> Result<(), Self::Error> {
         if self.serves_session_rows() {
-            pty_row_stub(channel, session).await;
+            pty_row_stub(channel, session);
         } else {
             decline_unfeatured_request(channel, session).await;
         }
@@ -450,7 +478,7 @@ impl russh::server::Handler for SessionHandler {
         session: &mut Session,
     ) -> Result<(), Self::Error> {
         if self.serves_session_rows() {
-            shell_row_stub(channel, session).await;
+            shell_row_stub(channel, session);
         } else {
             decline_unfeatured_request(channel, session).await;
         }
@@ -571,45 +599,57 @@ async fn decline_unfeatured_request(channel: ChannelId, session: &mut Session) {
 }
 
 /// The family-seam decline shape for a **routed-but-not-yet-served**
-/// row: fail the request and close the channel — the vanilla-clean
+/// row — now the spawned serving task's body: fail the request and
-/// shape for a serving path that would have owned the channel (wave 3
+/// close the channel, through the cloned handle the task was handed.
-/// replaces these stubs with the spawned serving paths; a declined
+/// The vanilla-clean shape for a serving path that owns the channel
-/// seam never leaks a half-served channel, since the decline IS the
+/// (wave 3 replaces these stubs with the real serving paths; a
-/// row this wave). Decline-first-alloc-never holds structurally:
+/// declined seam never leaks a half-served channel, since the decline
-/// nothing is allocated before the failure lands.
+/// IS the row this wave). Decline-first-alloc-never holds structurally:
-async fn seam_stub_decline(channel: ChannelId, session: &mut Session) {
+/// nothing is allocated before the failure lands — the spawn's handle
-    let _ = session.channel_failure(channel);
+/// clone is the only resource ever taken.
-    let _ = session.close(channel);
+async fn seam_stub_decline(handle: Handle, channel: ChannelId) {
+    let _ = handle.channel_failure(channel).await;
+    let _ = handle.close(channel).await;
+}
+
+/// Spawns the seam-stub decline as the row's serving path (ADR-006
+/// decision 4): the handle is cloned here, the decline runs on its own
+/// task, and the hook returns — no serving path awaits inline.
+fn spawn_seam_stub(channel: ChannelId, session: &Session) {
+    let handle = session.handle();
+    serve::spawn_serving_path(seam_stub_decline(handle, channel));
 }
 
 /// The git row's seam stub — wave 3 lands the alkgit seam here
 /// (`parse command → Service + repo → serve_duplex_resolved`), spawned
-/// per ADR-006 decision 4; the skeleton declines at the seam.
+/// per ADR-006 decision 4; the skeleton declines at the seam, on its
-async fn git_row_stub(channel: ChannelId, session: &mut Session) {
+/// own task.
-    seam_stub_decline(channel, session).await;
+fn git_row_stub(channel: ChannelId, session: &Session) {
+    spawn_seam_stub(channel, session);
 }
 
 /// The session pipe row's seam stub — wave 3 lands the alktty pipe
 /// seam here (`wrapper ++ [cmd]`, the OQ-08 argv append), spawned per
-/// ADR-006 decision 4; the skeleton declines at the seam.
+/// ADR-006 decision 4; the skeleton declines at the seam, on its own
-async fn pipe_row_stub(channel: ChannelId, session: &mut Session) {
+/// task.
-    seam_stub_decline(channel, session).await;
+fn pipe_row_stub(channel: ChannelId, session: &Session) {
+    spawn_seam_stub(channel, session);
 }
 
 /// The session pty row's seam stub — wave 3 lands the alktty
 /// `NegotiateRequest.tty` construction here
 /// (`drive_session_pre_negotiated`), spawned per ADR-006 decision 4;
-/// the skeleton declines at the seam.
+/// the skeleton declines at the seam, on its own task.
-async fn pty_row_stub(channel: ChannelId, session: &mut Session) {
+fn pty_row_stub(channel: ChannelId, session: &Session) {
-    seam_stub_decline(channel, session).await;
+    spawn_seam_stub(channel, session);
 }
 
 /// The session shell row's seam stub — wave 3 lands the alktty
 /// `NegotiateRequest.cmd` = serve-config shell argv here (OQ-08, no
 /// crate default), spawned per ADR-006 decision 4; the skeleton
-/// declines at the seam.
+/// declines at the seam, on its own task.
-async fn shell_row_stub(channel: ChannelId, session: &mut Session) {
+fn shell_row_stub(channel: ChannelId, session: &Session) {
-    seam_stub_decline(channel, session).await;
+    spawn_seam_stub(channel, session);
 }
 
 /// The tunnel row's seam stub: the explicit `ConnectFailed` reject

src/lib.rs

@@ -32,6 +32,7 @@ mod fingerprint;
 mod handler;
 mod host_key;
 mod key_source;
+mod serve;
 
 pub use adapter::SshAdapter;
 pub use config::{DialConfig, Limits, ServeConfig};
@@ -46,3 +47,9 @@ pub use fingerprint::fingerprint;
 pub use handler::SessionHandler;
 pub use host_key::{HostKeyPolicy, PresentedHostKey, RejectAll};
 pub use key_source::{HostKeySource, KeySource, VaultHostKeySource, VaultKeySource};
+/// Contract-test surface (`#[doc(hidden)]`): the spawn-and-return
+/// plumbing (`ADR-006 decision 4`, see `serve.rs`) — the mechanics the
+/// mechanics-proof rows in `tests/spawn_return.rs` compose over the
+/// real wire to pin the constraint as tests. Not documented API.
+#[doc(hidden)]
+pub use serve::{report_exit_status_then_close, spawn_serving_path};

src/serve.rs

@@ -0,0 +1,93 @@
+//! The spawn-and-return plumbing (`w2-spawn-return`) — ADR-006
+//! decision 4, the structural constraint of the whole spine, in code.
+//!
+//! russh at this pin runs server handler callbacks **inline inside the
+//! session's packet loop** — a blocking handler suspends all packet
+//! routing for the connection and deadlocks against the peer's
+//! immediate follow-up bytes (POC-3 proved by construction with
+//! receive-pack). The crate's answer is mechanical, not conventional:
+//!
+//! - **Every serving path is spawned** ([`spawn_serving_path`]) — the
+//!   handler hook clones the session handle
+//!   (`Session::handle()`), hands it into its serving future, spawns,
+//!   and returns. No serving path is ever awaited inline; the dispatch
+//!   seam stubs ([`crate::handler`]) hang off this helper, so the
+//!   wave-3 mappings inherit the pattern mechanically from the stubs.
+//! - **Exit/status reporting rides the cloned handle** — the spawned
+//!   task's only report surface is the handle clone it was handed;
+//!   [`report_exit_status_then_close`] preserves the wire ordering the
+//!   `mapping.md` `exit_status` row pins: `exit-status` sent **before**
+//!   close, from the spawned task.
+//!
+//! The *derivation contract's rules* (which exit code → which SSH exit
+//! shape, `mapping.md` §Exit chunk → SSH exit reporting rules 1–6) are
+//! wave-3 session-mapping material; this module is the mechanics only.
+//!
+//! ## Test surface
+//!
+//! Both helpers are re-exported `#[doc(hidden)]` from the crate root —
+//! the `handler.rs` §Test surface pattern: the mechanics-proof rows in
+//! `tests/spawn_return.rs` compose them over the real wire (a minimal
+//! proof handler standing in for the crate's own auth/dispatch hooks,
+//! which have their own contract suites) to pin the constraint as
+//! tests, not convention.
+
+use std::future::Future;
+
+use russh::ChannelId;
+use russh::server::Handle;
+
+/// Spawns one serving path on its own task (ADR-006 decision 4): the
+/// caller — the handler hook — clones the session handle, hands it
+/// into the serving future, calls this, and returns immediately.
+/// Nothing here is ever awaited inline: an inline serve deadlocks
+/// against the peer's follow-up bytes (POC-3 proved by construction).
+pub fn spawn_serving_path(serving: impl Future<Output = ()> + Send + 'static) {
+    tokio::spawn(serving);
+}
+
+/// The spawned task's exit report through the cloned handle:
+/// `exit-status(N)` then close — the ordering contract
+/// (`mapping.md`'s `exit_status` row: sent **before** close, from the
+/// spawned task). A failed send means the session is gone; nothing is
+/// reportable then.
+pub async fn report_exit_status_then_close(handle: &Handle, channel: ChannelId, exit_code: u32) {
+    let _ = handle.exit_status_request(channel, exit_code).await;
+    let _ = handle.close(channel).await;
+}
+
+#[cfg(test)]
+mod tests {
+    use std::sync::Arc;
+    use std::sync::atomic::{AtomicBool, Ordering};
+
+    use super::spawn_serving_path;
+
+    /// - *Contract stamp:* ADR-006 decision 4 (spawn-and-return: the
+    ///   handler call returns while the serving path is still running
+    ///   — handler-returns-fast is structural).
+    #[tokio::test]
+    async fn spawn_returns_while_the_serving_path_is_still_pending() {
+        let (release, waited) = tokio::sync::oneshot::channel::<()>();
+        let done = Arc::new(AtomicBool::new(false));
+        let flag = Arc::clone(&done);
+
+        spawn_serving_path(async move {
+            let _ = waited.await;
+            flag.store(true, Ordering::Release);
+        });
+
+        assert!(
+            !done.load(Ordering::Acquire),
+            "the spawn call returned while the serving path was still gated — \
+             no serving path is awaited inline"
+        );
+
+        release
+            .send(())
+            .expect("the gate releases exactly once, to the still-running path");
+        while !done.load(Ordering::Acquire) {
+            tokio::task::yield_now().await;
+        }
+    }
+}

tasks/implementation/w2-spawn-return.md

@@ -1,7 +1,7 @@
 ---
 id: w2-spawn-return
 name: Wave 2 — spawn-and-return plumbing + exit/status reporting mechanics
-status: pending
+status: completed
 depends_on: [w2-dispatch-decline-core]
 scope: narrow
 risk: medium
@@ -42,16 +42,16 @@ mechanically from the stubs this task leaves behind.
 
 ## Acceptance Criteria
 
-- [ ] Every serving path (the dispatch stubs) runs in a
+- [x] Every serving path (the dispatch stubs) runs in a
       `tokio::spawn`; no handler hook awaits a serving path inline
-- [ ] Exit/status reporting from spawned tasks rides the cloned
+- [x] Exit/status reporting from spawned tasks rides the cloned
       `Session::handle()`; `exit-status` is sent before channel close
-- [ ] A harness test proves the mechanics end to end: the client
+- [x] A harness test proves the mechanics end to end: the client
       observes `exit-status(N)` from a spawned stub that completed
       after the handler returned
-- [ ] A handler-returns-fast assertion pins the structural constraint
+- [x] A handler-returns-fast assertion pins the structural constraint
       as a test
-- [ ] `cargo test`, `cargo clippy --all-targets -- -D warnings`,
+- [x] `cargo test`, `cargo clippy --all-targets -- -D warnings`,
       `cargo fmt --check` pass
 
 ## References
@@ -67,8 +67,101 @@ mechanically from the stubs this task leaves behind.
 
 ## Notes
 
-> To be filled by implementation agent
+- `src/serve.rs` (new): the spawn-and-return plumbing as *mechanics*,
+  not convention — `spawn_serving_path(serving future)` (the one
+  choke point for the ADR-006 decision-4 spawn; nothing it takes is
+  ever awaited inline) and `report_exit_status_then_close(&Handle,
+  channel, N)` (the cloned-handle report with the `mapping.md`
+  `exit_status` row's ordering baked in: exit-status **before**
+  close). Both re-exported `#[doc(hidden)]` from the crate root — the
+  `SessionHandler` §Test-surface pattern — as the contract-test
+  surface the mechanics rows compose over the real wire. A unit row
+  (`spawn_returns_while_the_serving_path_is_still_pending` —
+  latch-gated future, spawn call returns first) pins spawn-then-return
+  directly. The derivation contract's *rules* (which exit code → which
+  SSH exit shape) stay wave-3 `mapping.md` material; no rule logic
+  landed here.
+- `src/handler.rs`: the four session-row seam stubs (`git_row_stub`,
+  `pipe_row_stub`, `pty_row_stub`, `shell_row_stub`) are now spawned
+  serving paths — `spawn_seam_stub` clones `session.handle()`,
+  `seam_stub_decline(handle, channel)` runs the failure-then-close
+  decline on the spawned task, and the hooks return without awaiting
+  anything. A compile-level fact coded around: the row stubs are
+  plain `fn`s (not `async`) — an `async fn` holding `&Session` across
+  the spawn makes the handler future non-`Send` (russh requires `+
+  Send` on handler futures; `Session` is not `Sync`), so the clone is
+  taken synchronously and only the handle moves into the spawned
+  future. Wire shapes are unchanged (all `w2-dispatch-decline-core`
+  rows pass unmodified with the spawned decline); the tunnel seam stub
+  and every open-level/capability decline stay inline by class — they
+  answer the open request *before* any channel is owned, so nothing
+  spawns yet (documented in the new handler module-doc section §The
+  spawn-and-return structural constraint; wave 3's
+  accept-only-after-fabric-open flow is where the tunnel row gains
+  its spawn).
+- `tests/spawn_return.rs` (new): the end-to-end mechanics proof over
+  the real wire — the russh server stack (duplex transport, harness
+  client/harness transport posture) mounted over a minimal proof
+  handler that is *not* the crate's `SessionHandler` (the auth and
+  dispatch hooks have their own suites; the proof subject is the
+  mechanics) and that serves execs by composing the exported
+  plumbing: `spawn_serving_path` of a release-gated serving future
+  reporting `exit-status(7)` + close through the cloned handle. Two
+  rows, each with a `Contract stamp:`:
+  - `handler_returns_fast_while_the_spawned_serving_path_is_pending` —
+    the structural constraint as a wire test: after the exec hook has
+    provably returned (a bounded wait on the handler-side timestamp;
+    the client's exec returns when the request is on the wire, not
+    when the hook ran) and with the serving path still gated, the
+    client's *follow-up traffic* (a second session channel + second
+    exec) is routed and dispatched normally — POC-3's follow-up-bytes
+    deadlock shape inverted — and the still-gated no-report state is
+    asserted before the release.
+  - `spawned_stub_reports_exit_status_before_close_after_the_handler_returned`
+    — the client observes `ExitStatus{7}` strictly before `Close`
+    (message-by-message, no draining that could hide ordering), and
+    the recorded timestamps prove the exit-status left *after* the
+    handler had already returned (the literal criterion).
+- **The proof-handler design note:** the gate is `tokio::sync::watch`-
+  based (no lost-wakeup race — a late subscriber sees the release,
+  which a `Notify`-with-`notify_waiters` shape could miss against
+  multiple spawned paths); the handler-side timestamps ride
+  `parking_lot::Mutex`.
+- Feature independence: the mechanics are feature-independent (the
+  helpers exist in every build and the proof rows run in every build
+  they were verified under — default, `git`, `session`, `tunnel`,
+  `sftp`, `--all-features`); no feature-gated code was added, so no
+  new decline-path coverage obligations arise.
 
 ## Summary
 
-> To be filled on completion
+Landed the spawn-and-return plumbing + exit/status reporting
+mechanics (`w2-spawn-return`) in three files:
+
+- **`src/serve.rs`** — `cargo test`-pinned mechanics: the
+  `spawn_serving_path` choke point (the ADR-006 decision-4 rule, in
+  code, never awaited inline) and `report_exit_status_then_close`
+  (exit-status before close through the cloned `Session::handle()`),
+  both `#[doc(hidden)]` contract-test re-exports; a unit row pins
+  spawn-returns-while-pending directly.
+- **`src/handler.rs`** — the four session-row seam stubs became
+  spawned serving paths (handle clone taken synchronously — the
+  non-`Send` shape that forcing `async fn` stubs would produce is
+  coded around and recorded); failure-then-close wire shapes unchanged
+  (all dispatch-core rows pass unmodified); open-level declines and
+  the tunnel seam stub stay inline by class, documented; a new
+  module-doc section carries the constraint and the wave-3
+  inherit-the-pattern note.
+- **`tests/spawn_return.rs`** — the end-to-end mechanics proof: two
+  `Contract stamp:` rows over the real wire (handler-returns-fast
+  under follow-up traffic with the spawn still pending; exit-status
+  before close, timestamps proving the report outlived the handler's
+  return), driven by a proof handler composing the exported plumbing.
+
+Verification (all green): `cargo test` · `cargo test
+--features {git,session,tunnel,sftp} --test dispatch_core` ·
+`cargo test --all-features` · `cargo clippy --all-targets -- -D
+warnings` · `cargo clippy --all-features --all-targets -- -D
+warnings` · `cargo fmt --check` · `cargo doc --no-deps` (no
+warnings). 93 lib/unit rows + the per-build dispatch matrices + the
+two mechanics rows across 7 test binaries.

tests/spawn_return.rs

@@ -0,0 +1,371 @@
+//! The spawn-and-return mechanics proof (`w2-spawn-return`) — the
+//! structural constraint (ADR-006 decision 4) pinned as tests, over
+//! the real wire.
+//!
+//! russh at this pin runs server handler callbacks inline inside the
+//! session's packet loop; a blocking handler deadlocks against the
+//! peer's follow-up bytes (POC-3 proved by construction with
+//! receive-pack). The crate's plumbing
+//! ([`alkssh::spawn_serving_path`],
+//! [`alkssh::report_exit_status_then_close`] — the `#[doc(hidden)]`
+//! contract-test re-exports from `src/serve.rs`) makes the escape
+//! mechanical: the handler hook clones `session.handle()`, spawns the
+//! serving future, and returns.
+//!
+//! These rows compose exactly that plumbing end to end: a minimal
+//! proof handler stands in for the crate's own auth/dispatch hooks
+//! (their correctness is `w2-auth-bridge`/`w2-dispatch-decline-core`
+//! contract territory; the `dispatch` seam stubs now spawn through the
+//! same plumbing, so the crate's own rows exercise the spawn too —
+//! `tests/dispatch_core.rs`). What is under test here is the
+//! *mechanics*: the spawn-then-return, the cloned-handle reporting,
+//! and the wire ordering — `exit-status` before close, arriving after
+//! the handler already returned, while the session's packet loop keeps
+//! routing the connection's follow-up traffic.
+
+#![allow(dead_code)]
+
+mod common;
+
+use std::sync::Arc;
+use std::time::Instant;
+
+use common::fixture;
+use common::harness::{
+    HarnessClientHandler, authenticate_publickey, client_config, next_message, open_session_channel,
+};
+use parking_lot::Mutex;
+use russh::keys::Algorithm;
+use russh::server::{Auth, ChannelOpenHandle, Session};
+use russh::{Channel, ChannelId, ChannelMsg, Disconnect, Error, MethodKind, MethodSet};
+
+use alkssh::Limits;
+
+/// The exit status the proof stub reports — an arbitrary distinct
+/// value the rows assert verbatim (`exit-status(7)`).
+const PROOF_EXIT: u32 = 7;
+
+/// The command the rows exec — anything; the proof handler ignores it.
+const PROOF_COMMAND: &str = "mechanics-proof-serving-path";
+
+/// The async completion gate the spawned serving paths hold on —
+/// released only after the rows have proven the handler returned.
+/// `watch`-based (no lost-wakeup race: a late subscriber still sees
+/// the release).
+fn serving_gate() -> tokio::sync::watch::Sender<bool> {
+    tokio::sync::watch::channel(false).0
+}
+
+/// The spawned task's wait: blocks until (or returns if) released —
+/// `borrow_and_update` marks the release seen so the wakeup is never
+/// lost or replayed.
+async fn gate_wait(rx: &mut tokio::sync::watch::Receiver<bool>) {
+    if !*rx.borrow_and_update() {
+        rx.changed().await.expect("the gate lives for the row");
+    }
+}
+
+/// What the proof handler records — the timestamps the rows compare.
+#[derive(Default)]
+struct ProofRecord {
+    handler_returned_at: Option<Instant>,
+    exit_reported_at: Option<Instant>,
+}
+
+type Shared = Arc<Mutex<ProofRecord>>;
+
+/// The proof handler (server side): mechanics-only — it authenticates
+/// the fixture alpha by the crate's one fingerprint path, accepts
+/// session channels, and serves every exec by *the crate's own
+/// spawn-and-return plumbing*: a serving path gated on the row's
+/// release, reporting `exit-status(7)` then close through the cloned
+/// handle.
+struct ProofServer {
+    gate: Arc<tokio::sync::watch::Sender<bool>>,
+    record: Shared,
+}
+
+impl russh::server::Handler for ProofServer {
+    type Error = Error;
+
+    async fn auth_publickey_offered(
+        &mut self,
+        _user: &str,
+        _offer: russh::server::PublicKeyOffer<'_>,
+    ) -> Result<Auth, Self::Error> {
+        Ok(Auth::Accept)
+    }
+
+    /// The mechanics rows' own fixture check — fingerprint equality
+    /// against fixture alpha through the crate's fingerprint path
+    /// (not the bridge contract; that is `w2-auth-bridge`'s suite).
+    async fn auth_publickey(
+        &mut self,
+        _user: &str,
+        key: russh::server::VerifiedPublicKey<'_>,
+    ) -> Result<Auth, Self::Error> {
+        let accepted = *key.algorithm == Algorithm::Ed25519
+            && alkssh::fingerprint(key.public_key) == fixture("alpha").fingerprint;
+        Ok(if accepted {
+            Auth::Accept
+        } else {
+            Auth::reject()
+        })
+    }
+
+    async fn channel_open_session(
+        &mut self,
+        _channel: Channel<russh::server::Msg>,
+        reply: ChannelOpenHandle,
+        _session: &mut Session,
+    ) -> Result<(), Self::Error> {
+        reply.accept().await;
+        Ok(())
+    }
+
+    /// The hook under proof: clone the handle, spawn the serving path
+    /// (gated — it stays pending past this method's return), record
+    /// the handler's return instant, and go.
+    async fn exec_request(
+        &mut self,
+        channel: ChannelId,
+        _data: &[u8],
+        session: &mut Session,
+    ) -> Result<(), Self::Error> {
+        let handle = session.handle();
+        let record = Arc::clone(&self.record);
+        let mut gate_rx = self.gate.subscribe();
+
+        alkssh::spawn_serving_path(async move {
+            gate_wait(&mut gate_rx).await;
+            alkssh::report_exit_status_then_close(&handle, channel, PROOF_EXIT).await;
+            record.lock().exit_reported_at = Some(Instant::now());
+        });
+
+        self.record.lock().handler_returned_at = Some(Instant::now());
+        Ok(())
+    }
+}
+
+/// One mounted mechanics-proof session: the duplex pair, the spawned
+/// server task (russh's own stack over the proof handler), and the
+/// connected client. Transport and posture mirror the loopback
+/// harness (its recorded decisions); the handler is intentionally not
+/// the crate's `SessionHandler` — the crate stack under a custom
+/// serving hook is the proof's subject.
+struct ProofSession {
+    client: russh::client::Handle<HarnessClientHandler>,
+    gate: Arc<tokio::sync::watch::Sender<bool>>,
+    record: Shared,
+    server_task: tokio::task::JoinHandle<Result<(), Error>>,
+}
+
+/// Mounts one proof session: the russh server stack over the proof
+/// handler, the harness client against it, the fixture alpha
+/// authenticated.
+async fn mount() -> ProofSession {
+    let gate = Arc::new(serving_gate());
+    let record: Shared = Arc::new(Mutex::new(ProofRecord::default()));
+
+    let config = Arc::new(russh::server::Config {
+        methods: MethodSet::from(&[MethodKind::PublicKey][..]),
+        keys: vec![fixture("bravo").private_key()],
+        inactivity_timeout: Some(Limits::default().inactivity_timeout),
+        ..russh::server::Config::default()
+    });
+
+    let (client_io, server_io) = tokio::io::duplex(common::harness::DUPLEX_CAPACITY);
+    let server = ProofServer {
+        gate: Arc::clone(&gate),
+        record: Arc::clone(&record),
+    };
+    let server_task = tokio::spawn(async move {
+        let running = russh::server::run_stream(config, server_io, server)
+            .await
+            .expect("the proof server mounts");
+        running.await
+    });
+
+    let policy = Arc::new(common::MemHostKeyPolicy::with_default(true));
+    let mut client = russh::client::connect_stream(
+        client_config(Limits::default()),
+        client_io,
+        HarnessClientHandler::new(Arc::clone(&policy)),
+    )
+    .await
+    .expect("the client mounts against the proof server");
+
+    let result = authenticate_publickey(&mut client, "mechanics-proof", fixture("alpha")).await;
+    assert!(
+        result.success(),
+        "the fixture identity authenticates before every mechanics row"
+    );
+
+    ProofSession {
+        client,
+        gate,
+        record,
+        server_task,
+    }
+}
+
+impl ProofSession {
+    /// Releases the gate — the spawned serving paths complete
+    /// asynchronously from here on.
+    fn release_gate(&self) {
+        let _ = self.gate.send(true);
+    }
+
+    /// Ends a row cleanly: disconnect, wait for the server task, and
+    /// require a clean session end (the harness posture).
+    async fn unmount(self) {
+        self.client
+            .disconnect(Disconnect::ByApplication, "row done", "en")
+            .await
+            .expect("client disconnects cleanly");
+        let outcome = self.server_task.await.expect("server task ends");
+        outcome.expect("clean session end");
+    }
+
+    /// Waits (bounded) until the exec hook has provably run and
+    /// returned — the client's `exec` returns when the request is on
+    /// the wire; the loop's processing of it is observed here.
+    async fn exec_hook_has_returned(&self) {
+        tokio::time::timeout(std::time::Duration::from_secs(5), async {
+            while self.record.lock().handler_returned_at.is_none() {
+                tokio::time::sleep(std::time::Duration::from_millis(2)).await;
+            }
+        })
+        .await
+        .expect("the exec hook returns promptly");
+    }
+
+    /// The handler provably returned while the spawned serving path
+    /// was still pending — the handler-returns-fast assertion.
+    fn assert_handler_returned_spawn_still_pending(&self, context: &str) {
+        let record = self.record.lock();
+        assert!(
+            record.handler_returned_at.is_some(),
+            "{context}: the exec hook must already have returned"
+        );
+        assert!(
+            record.exit_reported_at.is_none(),
+            "{context}: the spawned serving path is still gated — nothing reported yet"
+        );
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Rows.
+// ---------------------------------------------------------------------------
+
+/// - *Contract stamp:* ADR-006 decision 4 (the structural constraint:
+///   every serving path is spawned — the handler completes while the
+///   spawned task is still running) + mapping.md §The structural
+///   constraint: spawn-and-return (the follow-up-bytes deadlock
+///   inverted: the connection's loop keeps routing while a serving
+///   path is pending).
+#[tokio::test]
+async fn handler_returns_fast_while_the_spawned_serving_path_is_pending() {
+    let session = mount().await;
+
+    let mut channel = open_session_channel(&session.client)
+        .await
+        .expect("the session channel opens");
+    channel
+        .exec(false, PROOF_COMMAND)
+        .await
+        .expect("the exec request rides the wire");
+    session.exec_hook_has_returned().await;
+    session.assert_handler_returned_spawn_still_pending("after the exec");
+
+    // The follow-up-bytes inversion — POC-3's deadlock shape: the peer
+    // sends immediate follow-up traffic and the loop routes it while
+    // the first serving path is still pending.
+    let follow_up = open_session_channel(&session.client)
+        .await
+        .expect("the packet loop routes while the spawn is pending");
+    follow_up
+        .exec(false, PROOF_COMMAND)
+        .await
+        .expect("a second exec dispatches while the first serving path is pending");
+    session.assert_handler_returned_spawn_still_pending("after the follow-up traffic");
+
+    session.release_gate();
+
+    let first = next_message(&mut channel)
+        .await
+        .expect("the released serving path reports");
+    assert!(
+        matches!(
+            first,
+            ChannelMsg::ExitStatus {
+                exit_status: PROOF_EXIT
+            }
+        ),
+        "the spawned stub reports exit-status({PROOF_EXIT}), got {first:?}"
+    );
+
+    session.unmount().await;
+}
+
+/// - *Contract stamp:* mapping.md §exit_status row (sent **before**
+///   close, from the spawned task via `Session::handle()`) + ADR-006
+///   decision 4 (exit/status reporting rides the cloned handle).
+#[tokio::test]
+async fn spawned_stub_reports_exit_status_before_close_after_the_handler_returned() {
+    let session = mount().await;
+
+    let mut channel = open_session_channel(&session.client)
+        .await
+        .expect("the session channel opens");
+    let handler_returned_at = {
+        channel
+            .exec(false, PROOF_COMMAND)
+            .await
+            .expect("the exec request rides the wire");
+        session.exec_hook_has_returned().await;
+        let handler_returned_at = session
+            .record
+            .lock()
+            .handler_returned_at
+            .expect("the exec hook returned before this row observes anything");
+        session.release_gate();
+        handler_returned_at
+    };
+
+    // Strict wire order, message by message: exit-status first, close
+    // after — the ordering contract verbatim.
+    let first = next_message(&mut channel)
+        .await
+        .expect("the released serving path reports");
+    assert!(
+        matches!(
+            first,
+            ChannelMsg::ExitStatus {
+                exit_status: PROOF_EXIT
+            }
+        ),
+        "exit-status precedes close, got {first:?}"
+    );
+    let exit_reported_at = session.record.lock().exit_reported_at;
+
+    let second = next_message(&mut channel)
+        .await
+        .expect("the channel closes after its exit report");
+    assert!(
+        matches!(second, ChannelMsg::Close),
+        "close follows the exit report, got {second:?}"
+    );
+
+    // The literal criterion: the client observes exit-status that left
+    // the server *after* the handler had already returned.
+    let exit_reported_at = exit_reported_at.expect("the report timestamped");
+    assert!(
+        exit_reported_at > handler_returned_at,
+        "the spawned task reported exit-status after the handler returned \
+         (handler {handler_returned_at:?} vs report {exit_reported_at:?})"
+    );
+
+    session.unmount().await;
+}