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/003-protocol-v2-first.md

@@ -34,9 +34,10 @@ Decision drivers:
   exactly as observed in POC-1 (re-advertising between commands hangs real
   git).
 - Advertised capabilities are exactly what we serve: `ls-refs=unborn`,
-  `fetch=wait-for-done` (v1 fetch policy is full-closure pack on `done`;
-  multi-round negotiation, OQ-02, may add capability values when it
-  lands), `object-format=sha1` (+ sha256 under the feature flag, if and
+  `fetch=wait-for-done` (the full-closure pack on `done` policy
+  validated in POC-1/2; the multi-round ack loop, ADR-014, needs no
+  capability change — the acknowledgments section is grammar, not
+  capability), `object-format=sha1` (+ sha256 under the feature flag, if and
   when tested — OQ-05). Nothing unimplemented is advertised
   (shallow, filter, packfile-uris, object-info, server-option are
   *declined by omission*; POC-1 confirmed real git accepts this).
@@ -46,23 +47,22 @@ Decision drivers:
 - HTTP requests protocol V2 only; ssh requests protocol V2 only (see
   below).
 
-**V0/V1 policy: decided now as V2-only for v1.** Both doors
-(http and ssh) speak V2; clients that cannot speak V2 get a clear pkt-line
-error. Reversal is wire-visible (the advertised version set is a wire
+**V0/V1 policy: decided now as V2-only for v1 — fetch only.** Both doors
+(http and ssh) speak V2 for *fetch*; clients that cannot speak V2 get a
+clear pkt-line error. Reversal is wire-visible (the advertised version set is a wire
 format), so treat this as effectively one-way once published — the
 decision is still made now, and revisiting it needs a new ADR plus a
 deprecation window for clients. The motivation stands: a concrete consumer
 needing V0/V1 (e.g. very old CI images) has not been identified.
-The V0/V1 receive-pack shape (used by `git push` on http stateless framing
-in older clients) is unaffected by this choice: receive-pack is mostly
-version-independent (see transport.md).
+**Receive-pack is exempt**: the push path is V0-framed by upstream design
+on every client generation (no version negotiation exists on push at
+all — ADR-013 §1), so the V2-only rule never touches `git push`. The
+V0/V1 decline applies to the fetch path only.
 
-Sequencing note (fixes the scope contradiction the review found): the v1
-fetch policy that ships first is full-closure-on-`done` (POC-validated).
+Sequencing note: the v1 fetch policy that ships first is full-closure-on-`done` (POC-validated).
 Multi-round V2 negotiation (haves/acks without `done`) is in v1's *scope*
-but lands after the done-path works end-to-end; its ack logic is OQ-02's
-open question, and any capability-advertisement change it requires
-happens then.
+but lands after the done-path works end-to-end; its ack loop is now
+decided (ADR-014) and needs no capability-advertisement change.
 
 ## Consequences