refactor(architecture): ADR-010 — pure protocol crate (alktty template)

Structural decision (OQ-09 resolved): alkgit follows the alktty/
alktunnels template — a single published protocol crate on alkcall
channels, no binary, no front doors.

- ADR-010 supersedes ADR-001 (crate decomposition) and ADR-006
  (http router factory); both marked Superseded
- Single crate at repo root: Cargo.toml with gix feature (default-on
  backend implementations; wire layer compiles without it —
  gix-hash always-on with sha1 per the compile-time-rejected
  invariant), crates/ workspace deleted, src/lib.rs stub in place
- doors.md replaces http.md/ssh.md/alkgitd.md: alkhttp git-feature
  sequencing (after first publish), alkssh requirement (fixed-grammar
  exec dispatch), native alk/git path, downstream assembly
- backend.md replaces storage.md: GitRegistry/GitRefs/GitPackGen/
  GitPackIngest traits (ingest validates, refs commits — single CAS
  home), gix feature encodes POC-2 prerequisites
- transport.md reframed for the single crate; backend traits replace
  hook traits in the public API
- OQ-09 resolved (all five sub-decisions in ADR-010), OQ-01 resolved
  (subsumed), OQ-03 narrowed to publish-freeze, OQ-08 narrowed to
  registry identity + vault placement, OQ-07 rescoped to the gix
  feature's registry impl
- vision.md v2: single-binary/monorepo framing corrected as
  init-agent artifact; POC checklist marked complete
- AGENTS.md + .opencode agent specs updated to the new shape

Verification: cargo build (default + no-default-features), cargo test
--all-features, clippy --all-features -D warnings, fmt --check all
pass. Third review round: zero critical, all warnings/suggestions
addressed (GitPackGen signature amended in ADR-004, stale anchors
fixed, ADR-006 body tense normalized, CAS split stated, vision
residuals cleaned).

docs/architecture/transport.md

@@ -3,28 +3,34 @@ status: draft
 last_updated: 2026-09-21
 ---
 
-# alkgit-transport: Git Smart Protocol
+# alkgit: Git Smart Protocol (wire layer)
 
 ## What it is
 
-The git protocol layer: pkt-line session substrates, protocol V2 state
-machines (advertisement, ls-refs, fetch, receive-pack), and the glue that
-keeps adapters from touching `futures_io` or raw packetline APIs. Depends
-on `alkgit-core` and alkcall types only (ADR-001/002).
+The wire half of the `alkgit` protocol crate: pkt-line session substrates,
+protocol V2 state machines (advertisement, ls-refs, fetch, receive-pack),
+and the substrate types that keep doors and handlers from touching
+`futures_io` or raw packetline APIs. Depends on alkcall and — only under
+the `gix` feature — the backend implementations (ADR-010). The wire layer
+itself is backend-trait-only, which is what makes `default-features =
+false` compile without gix.
 
 ## Substrate layer (ADR-005)
 
 Two session entry points over one state-machine core. ADR-005 owns the
 full decision (what each substrate owns and why); the surface is:
 
-- **Duplex session** (ssh, git://, embedded stream doors) — input:
-  (peer identity, resolved repo id, duplex stream, `Limits`). Encapsulates
-  the split/compat/packetline bridge, the request reader (delim-aware
-  parsing, `reset()` discipline, break-on-error), and the sideband writer.
-- **Stateless session** (smart-http) — input: (peer identity, resolved
-  repo id, request-reader, response-writer, `Limits`) per http POST; adds
-  the http-framing rules (capability-dump skip, flush-only responses,
-  probe handling).
+- **Duplex session** (the `alk/git` ALPN producer, channels-opened git
+  sessions, embedder stream doors) — input: (peer identity, resolved repo
+  id, duplex stream, `Limits`). Encapsulates the split/compat/packetline
+  bridge, the request reader (delim-aware parsing, `reset()` discipline,
+  break-on-error), and the sideband writer.
+- **Stateless session** (smart-http doors, e.g. alkhttp's future `git`
+  feature) — input: (peer identity, resolved repo id, request-reader,
+  response-writer, `Limits`) per http POST; adds the http-framing rules
+  (capability-dump skip, flush-only responses, probe handling). IO-abstract:
+  the door supplies reader/writer; see [doors.md](doors.md) for the
+  mounting.
 
 Both substrates feed the same V2 state machines; statelessness is a
 substrate property (per-request state), not a protocol fork.
@@ -40,12 +46,15 @@ substrate property (per-request state), not a protocol fork.
   `fetch=wait-for-done` for the v1 done-path policy — ADR-003 pins the
   values; OQ-02 may extend them when multi-round lands,
   `object-format=sha1`). Unimplemented features are declined by omission
-  (validated against real git, POC-1).
+  (validated against real git, POC-1). `git-upload-archive` is not
+  served (fixed refusal — ADR-008's never-execute rule, ssh analog in
+  doors.md).
 
 ### ls-refs
 
 - Parse `command=ls-refs` (peel, symrefs, ref-prefix), stream ref lines
-  from core's listing, flush. `ref-prefix` filtering is client-driven.
+  from the backend's listing, flush. `ref-prefix` filtering is
+  client-driven.
 
 ### fetch
 
@@ -53,10 +62,10 @@ substrate property (per-request state), not a protocol fork.
   POC-1 to-do).
 - Negotiation policy (v1 initial): full-closure pack on `done`
   (POC-validated). Multi-round ack/NAK negotiation: **OQ-02**.
-- Pack generation via core (ADR-004), streamed over sideband on duplex /
-  sideband-in-response on http; generation runs on `spawn_blocking` with
-  the owned odb handle moved in (POC-2's shape: store shared, handle per
-  session, generation on blocking threads).
+- Pack generation via `GitPackGen` (ADR-004), streamed over sideband on
+  duplex / sideband-in-response on http; generation runs on
+  `spawn_blocking` with the owned handle moved in (POC-2's shape: store
+  shared, handle per session, generation on blocking threads).
 - Round/haves budgets enforced here (ADR-009).
 
 ### receive-pack (push)
@@ -66,8 +75,9 @@ substrate property (per-request state), not a protocol fork.
   transactions, emit status report. The capability set, shallow policy,
   CAS timing, and exact status-report shape are OQ-04's investigation
   target (design intent: shape per OQ-04's resolution).
-- Validation pending — **OQ-04** (includes the receive-pack version
-  surface, which ADR-003's fetch-V2 decision does not cover).
+- Ingestion via `GitPackIngest` (ADR-004); validation pending — **OQ-04**
+  (includes the receive-pack version surface, which ADR-003's fetch-V2
+  decision does not cover).
 
 ### Error taxonomy
 
@@ -83,10 +93,11 @@ round, receive-pack max size, wall clock, sideband chunk size (fixed
 
 ## Public API surface (v1)
 
-`lib.rs` re-exports: `Session` (duplex) + `StatelessRequest` (http)
-entry points, `Limits`, hook trait(s) connecting to core
-(advertisement data, pack generation, ingestion), protocol error enums.
-The exact embedder-facing freeze point: **OQ-03**.
+Crate-root re-exports (alktty pattern; the full list in
+[backend.md](backend.md) §public API): `GitAdapter` + `register_openable`,
+`GitSession`, substrate types, `Limits`, the backend traits (ADR-010's
+seam), protocol error enums; gix-feature types under the feature.
+Publish-freeze point: **OQ-03**.
 
 ## Design Decisions
 
@@ -97,18 +108,22 @@ The exact embedder-facing freeze point: **OQ-03**.
 | [004](decisions/004-pack-pipeline.md) | Pack pipeline | generation on blocking threads, O(counts) |
 | [005](decisions/005-session-substrate-types.md) | Substrate types | request reader, sideband sink, http framing rules |
 | [009](decisions/009-bounded-resources-budget.md) | Budgets | `Limits` in every session tuple |
+| [010](decisions/010-pure-protocol-crate.md) | Pure protocol crate | wire layer is backend-trait-only |
 
 ## Open Questions
 
 - **OQ-02**: V2 multi-round negotiation (open — efficiency, not
   correctness).
 - **OQ-04**: receive-pack state machine + validation (deferred(unclear)).
-- **OQ-03**: embedder API freeze (deferred(scope)).
+- **OQ-03**: publish/API freeze (partially resolved — single-crate shape
+  settled by ADR-010).
 - **OQ-05**: sha256 policy (deferred(scope)).
 
 ## References
 
-- `docs/research/poc-1-findings.md`, `docs/research/poc2-findings.md`, `docs/research/poc3-findings.md` (the
-  normative wire behavior — observed against real git, not docs' grammar)
+- `docs/research/poc-1-findings.md`, `docs/research/poc2-findings.md`,
+  `docs/research/poc3-findings.md` (the normative wire behavior —
+  observed against real git, not docs' grammar)
 - `docs/research/git-protocol.md` (inventory + observed corrections)
-- `docs/research/gitoxide.md` §"Wire format" (packetline contracts)
+- `docs/research/gitoxide.md` §"Wire format" (packetline contracts)
+- alktty `wire.rs`/`session.rs`/`adapter.rs` (the template's half shapes)