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.

Cargo.toml

@@ -38,6 +38,11 @@ sha256 = ["gix-hash/sha256"]
 
 [dependencies]
 alkcall = "0.8"
+# Trait-family dyn-compatibility (ADR-012 §1: all backend traits are
+# #[async_trait] — bare `async fn` in a trait is not dyn-compatible,
+# review 001 A-2; the family-wide pattern: alktty TtyBackend, alkcall
+# ProtocolHandler).
+async-trait = "0.1"
 gix-odb = { version = "0.84", optional = true, default-features = false, features = ["sha1"] }
 gix-pack = { version = "0.74", optional = true, features = ["sha1"] }
 gix-ref = { version = "0.67", optional = true }