docs(arch): alknet-tty spec sanity-check fixes — TtyControlHandle newtype (C1), backend cleanup contract (W1, ADR-056), clarifications (S1-S5)

C1: Box<dyn TtyControl + Clone> does not compile (Clone is not object-safe).
Replace with a TtyControlHandle newtype: #[derive(Clone)] wrapping
Arc<dyn TtyControl + Send + Sync>. The trait stays object-safe; the
newtype carries the Clone-ability. Updated across tty-backend.md,
ADR-053, OQ-43, and the tty README.

W1: 'Drop is the cleanup, threads exit on channel close' was false for
the local PTY waiter thread (blocked in Child::wait(), does not observe
channel close) — a child that ignores stdin EOF would be orphaned on
session cancel. New ADR-056 commits a backend cleanup contract: dropping
the exit_code future MUST kill the session target. The local backend
implements it via a ChildKiller held in the exit_code future's Drop
guard (disarmed on resolve). Corrected the lifecycle section in
tty-adapter.md, added the mechanism + constraint to tty-local.md, and
referenced ADR-056 from tty-backend.md, overview.md, both READMEs.

S1: error-frame < 16 MiB stated as a wire-format invariant (not an
assumption) so the 0x00-first-byte disambiguation trick is sound by
construction.
S2: carriage field validation (MUST be "raw" in v1, else
malformed_negotiation) specified.
S3: NegotiateRequest + TerminalParamsWire Rust structs added with
serde defaults and validation rules.
S4: modes field annotated 'backends MUST ignore content in v1' in both
tty-backend.md and ADR-053.
S5: AsyncWrite/Stream qualified with crate origins (tokio::io,
futures_core) in the TtyHandle struct definitions.

docs/architecture/README.md

@@ -22,7 +22,7 @@ The alknet-call crate is **implemented and reviewed** — both the server-side c
 
 **Next step**: The storage/repo-pattern ADRs (030–033) are accepted and amend the core and call specs. The next implementation phase is the ADR-029 migration (peer-keyed overlays, `PeerRef` routing, retire `remote_safe`/`trusted_peer`) with the ADR-030 `PeerEntry` change and the ADR-032 `forwarded_for` field folded in — the `OperationContext`, `from_call` handler, and `AuthPolicy` are all under edit, making this the cheapest window. After that: alknet-http implementation (specs drafted; `h3`/WebTransport deferred per ADR-044, browser bidirectional path uses WebSocket), which consumes the `CredentialStore` trait and the `OperationAdapter` contract. The alknet-ssh crate (the other post-core crate, specced in parallel) proceeds independently — it depends on `alknet-core`, not `alknet-call`.
 
-**alknet-tty specs drafted.** The alknet-tty crate (terminal session protocol handler — `ProtocolHandler` on `alknet/tty`, two-carriage wire format with a raw chunk codec + JSON control channel, backend-agnostic via a `TtyBackend` trait) now has architecture specs: [crates/tty/](crates/tty/) (overview, tty-wire, tty-backend, tty-adapter, tty-local) and four ADRs — [ADR-052](decisions/052-alknet-tty-wire-format-and-two-carriage.md) (wire format: `alknet/tty` ALPN, JSON negotiation frame then raw chunks, fixed channel set 0-3, control as JSON), [ADR-053](decisions/053-ttybackend-trait-and-ttyhandle.md) (`TtyBackend` trait + `TtyHandle`; `exit_code` as a `Future`; backends need not be natively async — REQ-TTY-01 from the local-PTY POC), [ADR-054](decisions/054-local-tty-backend-sibling-crate.md) (`alknet-tty-local` sibling crate behind a `local` feature re-export; PTY vs pipe per-session; the runner pattern preserved), [ADR-055](decisions/055-exit-code-on-control-chunk.md) (exit code on a stream_type 3 control chunk; "exit chunk is last" invariant; adapter owns the ordering). The specs are grounded in the alknet-docker POC (`docs/research/alknet-docker/poc-summary.md`) and the alknet-tty POC (`/workspace/alknet-tty-poc/`, built 2026-07-05), which validated the wire format, the control channel, the local-PTY bridge, and the signal-delivery contract (REQ-TTY-02). The docker and SSH backends are future crates that implement the `TtyBackend` trait — out of scope for this spec set, but the trait shape is committed so they can be built against it.
+**alknet-tty specs drafted.** The alknet-tty crate (terminal session protocol handler — `ProtocolHandler` on `alknet/tty`, two-carriage wire format with a raw chunk codec + JSON control channel, backend-agnostic via a `TtyBackend` trait) now has architecture specs: [crates/tty/](crates/tty/) (overview, tty-wire, tty-backend, tty-adapter, tty-local) and five ADRs — [ADR-052](decisions/052-alknet-tty-wire-format-and-two-carriage.md) (wire format: `alknet/tty` ALPN, JSON negotiation frame then raw chunks, fixed channel set 0-3, control as JSON), [ADR-053](decisions/053-ttybackend-trait-and-ttyhandle.md) (`TtyBackend` trait + `TtyHandle`; `exit_code` as a `Future`; backends need not be natively async — REQ-TTY-01 from the local-PTY POC; `TtyControlHandle` newtype for `Clone`-ability — `Clone` is not object-safe), [ADR-054](decisions/054-local-tty-backend-sibling-crate.md) (`alknet-tty-local` sibling crate behind a `local` feature re-export; PTY vs pipe per-session; the runner pattern preserved), [ADR-055](decisions/055-exit-code-on-control-chunk.md) (exit code on a stream_type 3 control chunk; "exit chunk is last" invariant; adapter owns the ordering), [ADR-056](decisions/056-backend-cleanup-on-session-cancel.md) (backend cleanup contract: dropping the `exit_code` future on session cancel MUST kill the session target — closes the orphaned-process gap the local-PTY POC surfaced for the waiter thread). The specs are grounded in the alknet-docker POC (`docs/research/alknet-docker/poc-summary.md`) and the alknet-tty POC (`/workspace/alknet-tty-poc/`, built 2026-07-05), which validated the wire format, the control channel, the local-PTY bridge, and the signal-delivery contract (REQ-TTY-02). The docker and SSH backends are future crates that implement the `TtyBackend` trait — out of scope for this spec set, but the trait shape is committed so they can be built against it.
 
 ## Architecture Documents
 
@@ -117,6 +117,7 @@ The alknet-call crate is **implemented and reviewed** — both the server-side c
 | [053](decisions/053-ttybackend-trait-and-ttyhandle.md) | TtyBackend Trait and TtyHandle — the Backend Inversion Point | Accepted |
 | [054](decisions/054-local-tty-backend-sibling-crate.md) | Local TTY Backend as a Sibling Crate (`alknet-tty-local`) | Accepted |
 | [055](decisions/055-exit-code-on-control-chunk.md) | Exit Code on a Control Chunk (the Last Chunk Before Stream Close) | Accepted |
+| [056](decisions/056-backend-cleanup-on-session-cancel.md) | Backend Cleanup on Session Cancel (Drop of `exit_code` Kills) | Accepted |
 
 ## Open Questions