docs(architecture): resolve OQ-04 — receive-pack state machine (ADR-013)

- ADR-013: V0-framed push machine grounded in real git 2.43.0 captures
  (file://, git://, smart-http mock, raw stdio into real receive-pack):
  V0-shaped ref advertisement (caps on first ref line, capabilities^{}
  sentinel only for empty repos), served capability set, shallow requests
  rejected for v1, thin packs accepted with server-odb bases (no
  capability involved; push.thin default), ingestion bound to
  Bundle::write_to_directory_eagerly + gix-fsck + one gix-ref transaction
  per push (.keep-guarded), unpack-first CAS timing with observed
  upstream order, band-1 pkt-line-framed status report, http framing
  (probe/Content-Length/chunked), v1 update policy (CAS only; deletes
  and force-push allowed)
- docs/research/push-captures.md: the normative push wire record
- transport.md/backend.md/doors.md: receive-pack sections rewritten to
  the decided shapes; backend.md ingestion composition bound; stale
  OQ-04 references resolved
- ADR-003 amended: V2-only governs fetch; push is V0-framed by upstream
  design (fixes the V2-only contradiction found in review)
- ADR-009 amended: haves default reconciled with the client's stateless
  ceiling (16384); blocking-pipeline budget covers generation+ingestion
- OQ-04 resolved; tracker task closed; CAS-fail-fast optimization
  tracked (tasks/architecture/oq-13-cas-failfast.md)
- research index: poc findings + capture docs listed

Verification: cargo test / clippy -D warnings / fmt --check / doc pass

docs/architecture/decisions/004-pack-pipeline.md

@@ -52,9 +52,9 @@ band), never emit a broken pack — check entry statistics
 **Receive-side ingestion** parses the client pack stream
 (`gix-pack::data::input` with `streaming-input`), fscks it
 (`gix-fsck` connectivity), and applies ref updates via `gix-ref`
-transaction CAS. The exact push state machine (capability advertisement
-set, CAS timing, status report, http framing) is OQ-04's investigation
-target; the ingestion-tool choice above is decided.
+transaction CAS. The push state machine (capability advertisement
+set, CAS timing, status report, http framing) is decided in ADR-013;
+the ingestion-tool choice above is decided.
 
 **Delta synthesis** for loose objects is an optimization backlog item, not
 a v1 commitment: existing pack deltas copy through for free; loose objects