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/decisions/013-receive-pack-state-machine.md

@@ -115,7 +115,9 @@ verification of the ingestion composition. Captures are recorded in
      concurrency budget apply (the generation budget covers ingestion —
      same `spawn_blocking` pool, amended in ADR-009's table).
    - Ingestion runs on `spawn_blocking` (POC-2's shape: store shared,
-     handle per session).
+     handle per session — inside the gix impl; the wire layer's
+     pipeline-concurrency permit wraps the trait call per ADR-009,
+     backend.md concurrency model).
    - fsck/connectivity: `gix-fsck::Connectivity` over the odb handle
      after indexing, for each new tip reachable from the pushed refs;
      missing objects → `unpack ng <reason>`.