docs(arch): A-2 + A-6 — async-trait trait family, pinned execution model

Resolves review 001 findings A-2 (critical) and A-6 (major) — the same
signature surface:

- ADR-012 §1: registry traits amended to #[async_trait] (bare async fn
  in traits is not dyn-compatible, E0038; ops sit behind Arc<dyn
  GitRegistryStore>). Desugared boxed Future form pinned in OQ-03's
  freeze inventory. async-trait = "0.1" added to the manifest.
- backend.md concurrency model: the five-trait family is
  #[async_trait] Send + Sync dyn-compatible; the wire layer enforces
  ADR-009's pipeline-concurrency budget itself (permit acquired around
  each GitPackGen/GitPackIngest call — the concrete admission point);
  impls must not block the async executor and own their internal
  threading (gix impls run spawn_blocking inside the impl — POC-2's
  shape restated at its true layer).
- transport.md §fetch: spawn_blocking sentence rephrased to the
  trait-contract version (the wire spec stops speaking gix).
- ADR-009: enforcement point of the blocking-pool budget made concrete.
- ADR-013 §6: ingestion spawn_blocking line aligned.
- review 001: A-2, A-6 marked resolved.

Verification: cargo test, clippy -D warnings, fmt --check, doc
--no-deps, check --no-default-features, check --all-features — all
clean.

docs/architecture/transport.md

@@ -1,6 +1,6 @@
 ---
 status: draft
-last_updated: 2026-09-25
+last_updated: 2026-09-29
 ---
 
 # alkgit: Git Smart Protocol (wire layer)
@@ -70,9 +70,10 @@ substrate property (per-request state), not a protocol fork.
   re-sends wants + commons each round — negotiation-captures.md).
   Advertisement text is unchanged: `fetch=wait-for-done`.
 - 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).
+  duplex / sideband-in-response on http; the call is async-trait, with
+  the pipeline-concurrency permit acquired around the call by the wire
+  layer (ADR-009's enforcement point — backend.md concurrency model;
+  the gix impl's internal `spawn_blocking` is its own detail).
 - Round/haves budgets enforced here (ADR-009); an empty resulting pack
   (client already has everything) is a valid zero-object packfile.