docs(arch): review-001 doc batch — A-4, D-2, D-3, N-1, N-2

Amendment batch (no new decisions, all doc-level):

- A-4: done-round boundary set is the recognized subset — request
  haves filtered through common_haves, the same honest-boundary rule
  as the ack rounds (never honor an unverified have); amendment clause
  in ADR-014 §2, same rule restated in transport.md §fetch.
- D-2: amendment note on ADR-007 step 3 — the per-repo check is
  ADR-011's authorize policy function (static ACL engine fails closed
  on None identity); step order unchanged.
- D-3: authorized-repo marker added to both substrate input tuples in
  transport.md and to backend.md's public-API list (ADR-007's
  type-level enforcement promise is now findable from the transport
  spec).
- N-1: advertisement ref cap is fail-closed (breach is an error, never
  a silent truncation) — transport.md §Limits.
- N-2: RegistryError::NotFound and authorization failure collapse to
  the same wire error at the variant→wire mapping — transport.md
  §error taxonomy.
- review 001: A-4/D-2/D-3/N-1/N-2 marked resolved.

Verification: cargo doc --no-deps, cargo test — clean.

docs/architecture/backend.md

@@ -127,7 +127,10 @@ Two independent seams, two default-on features:
 
 Crate-root re-exports (the alktty pattern): backend traits + types
 (including `RepoRecord`, `AccessAction`, the `authorize` policy
-function, and the `git/repo/*` op spec+handler pairs), `GitAdapter`/
+function, the authorized-repo marker type (the door constructs it only
+after the ADR-007 check passes — session tuples carry it, transport.md
+substrate layer), and the `git/repo/*` op spec+handler pairs),
+`GitAdapter`/
 `register_openable` (producer), `GitSession` (consumer), substrate
 types, `Limits`, protocol error enums; feature types (`GixBackend`
 family under `gix`, the file registry under `registry-file`) exported