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  (http router factory); both marked Superseded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/backend.md

@@ -0,0 +1,103 @@
+---
+status: draft
+last_updated: 2026-09-21
+---
+
+# Backend traits: the storage seam
+
+## What this is
+
+The payload side of the protocol crate: the traits a service deployer
+implements (or consumes via the feature-gated gix implementation) so the
+git protocol can talk to storage. Per ADR-010 this mirrors alktty's
+`TtyBackend` pattern — traits in-crate, real implementation behind a
+feature. Unlike alktunnels (no backend trait), git needs the seam: pack
+generation/ingestion is too heavy to hard-wire.
+
+## The trait family (ADR-010 sub-decision 4)
+
+Four traits, kept small and orthogonal — the protocol crate never sees
+gix types:
+
+1. **`GitRegistry`** — repo id → (storage root, visibility, ACL scope).
+   The authoritative mapping (ADR-008); resolution failure is
+   indistinguishable from authorization failure (ADR-007). Metadata holds
+   vault references, never secrets.
+2. **`GitRefs`** — listing for advertisement (refs + peeled tags + symref
+   targets, the ls-refs response data) and ref transactions (CAS apply
+   for receive-pack, name validation per git ref rules + reserved-
+   namespace deny-list).
+3. **`GitPackGen`** — (repo, wants, haves, limits) → streaming pack
+   (`io::Write` consumer). Negotiation-agnostic. Missing objects abort
+   with an error, never a broken pack (ADR-004).
+4. **`GitPackIngest`** — client pack stream → indexed pack + fsck/
+   connectivity report. It *prepares* the validated ref updates; the
+   transaction itself is applied by `GitRefs` (single CAS home — ingest
+   validates, refs commits). Budgeted (ADR-009 max pack size);
+   blocking-thread friendly.
+
+Minimal-vs-full was the open sub-question; resolved as **full family** —
+the four traits are each one or two methods plus types, and collapsing
+them (e.g. refs into the registry) would force one impl block per
+downstream where independent seams are cheaper to satisfy. The `gix`
+feature implements all four; a downstream with its own object store
+implements 3–4 and reuses 1–2, or none of it.
+
+## The gix feature (default on)
+
+- `alkgit = { default-features = true }` — wire layer + gix backend;
+  `default-features = false` — wire/protocol layer only (an embedder
+  brings its own backend). Hash: `sha1` pinned (the compile-time-rejected
+  invariant from `docs/research/gitoxide.md`); `sha256` passthrough
+  feature (OQ-05 policy unchanged).
+- Encodes the POC-2 prerequisites by construction: odb handle sharing
+  (`Arc<Store>` shared, per-session handles, `prevent_pack_unload()` +
+  `ignore_replacements = true`), generation on blocking threads,
+  O(counts) memory, missing-objects abort.
+- Received-pack ingestion via `gix-pack::data::input` (`streaming-input`)
+  + `gix-fsck` + `gix-ref` transactions (ADR-004). Validation against
+  real `git push` is OQ-04.
+
+## Concurrency model
+
+- `gix` structures: `parking_lot` short-held locks; per-session handles
+  moved into `spawn_blocking` tasks (POC-2's shape: store shared, handle
+  per session, generation on blocking threads).
+- Poisoned locks: `unwrap_or_else(|e| e.into_inner())` (convention 2).
+- The traits are `Send + Sync` object-safe; impls run under the
+  adapter's tokio context.
+
+## Public API surface
+
+Crate-root re-exports (the alktty pattern): backend traits + types,
+`GitAdapter`/`register_openable` (producer), `GitSession` (consumer),
+substrate types, `Limits`, protocol error enums; `gix`-feature types
+(`GixBackend`-family) exported under the feature. The embedder-facing
+freeze point remains OQ-03 (narrowed: it is now this crate's own publish,
+not a multi-crate freeze).
+
+## Design Decisions
+
+| ADR | Decision | Summary |
+|---|---|---|
+| [004](decisions/004-pack-pipeline.md) | Pack pipeline | `data::output` gen / `data::input` ingestion |
+| [007](decisions/007-acl-before-advertisement.md) | ACL first | registry returns rule inputs |
+| [008](decisions/008-registry-resolved-repo-identity.md) | Repo identity | wire names are registry ids |
+| [009](decisions/009-bounded-resources-budget.md) | Budgets | limits flow into gen/ingest |
+| [010](decisions/010-pure-protocol-crate.md) | Pure protocol crate | traits in-crate, gix behind a feature |
+
+## Open Questions
+
+- **OQ-06**: registry backing store (deferred(scope) — the trait is what
+  matters; the gix feature can ship a config-file/classic-on-disk impl,
+  and richer backing is downstream's choice).
+- **OQ-04**: pack ingestion validation (deferred(unclear)).
+- **OQ-05**: sha256 policy (deferred(scope)).
+
+## References
+
+- `docs/research/gitoxide.md` (API contract notes — normative for the
+  gix impl)
+- `docs/research/poc2-findings.md` (generation pipeline + prerequisites)
+- alktty `backend.rs`/`local` module (the trait + feature template)
+- ADR-010 (the structural decision)