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

tasks/architecture/oq-04-receive-pack.md

@@ -1,42 +1,45 @@
 ---
 id: architecture/oq-04-receive-pack
 name: OQ-04 unblock — receive-pack walkthrough/POC arrives
-status: pending
+status: completed
 depends_on: []
 scope: narrow
 risk: trivial
 impact: component
 level: research
-tags: [external-trigger, deferred-oq]
+tags: [external-trigger, deferred-oq, resolved-early]
 ---
 
 ## Description
 
 Tracker for OQ-04 (`deferred(unclear)`): receive-pack (push) design
-validation. This task is not actionable work — it tracks whether the
-investigation has arrived. When a receive-pack design walkthrough (against
-real `git push` captures) or a validation push POC is scheduled, mark this
-completed; OQ-04 transitions to `open`. The investigation scope is in
-`docs/architecture/open-questions.md` (OQ-04): capability advertisement
-set, request-line parsing + shallow policy, thin-pack acceptance, CAS
-timing, status report, http framing.
+validation. **Resolved early (2026-09-25, ADR-013)** — the investigation
+was run in-session rather than waiting for scheduling: real `git push`
+captures (git 2.43.0 over file://, git://, smart-http, plus raw stdio
+requests into real `git receive-pack`) resolved the full push state
+machine. OQ-04 is resolved by
+`docs/architecture/decisions/013-receive-pack-state-machine.md`; captures
+in `docs/research/push-captures.md`.
 
 ## Work
 
 None by default. On trigger: set OQ-04 to `open` in
 `docs/architecture/open-questions.md` and spawn the focused session that
-works through the push state machine.
+works through the push state machine. (Triggered and completed in the
+2026-09-25 session.)
 
 ## Verification
 
 - OQ-04 status matches this task's state in
-  `docs/architecture/open-questions.md`.
+  `docs/architecture/open-questions.md` (resolved).
 
 ## Out of scope
 
-- Actually designing/implementing receive-pack (that is the follow-on
-  session's work).
+- Actually implementing receive-pack (phase 3 implementation work).
 
 ## Summary
 
-> Filled on completion.
+Resolved early via ADR-013: the walkthrough ran in-session; all seven
+unknowns (advertisement set, shallow policy, thin-pack acceptance,
+ingestion composition, CAS timing, status report, version/framing
+surface) have capture-grounded decisions.

tasks/architecture/oq-13-cas-failfast.md

@@ -0,0 +1,45 @@
+---
+id: architecture/oq-13-cas-failfast
+name: Backlog — CAS-before-unpack fail-fast pre-check on push
+status: pending
+depends_on: []
+scope: narrow
+risk: low
+impact: component
+level: implementation
+tags: [optimization-backlog]
+---
+
+## Description
+
+Optimization backlog item from ADR-013 (§7, §Consequences): a pre-unpack
+CAS pre-check on receive-pack — reject commands with obviously-wrong
+old-ids before reading the pack body, failing fast instead of ingesting
+a pack whose refs would be rejected anyway. The protocol-correct order
+(unpack-first, then per-ref checks) is what v1 implements; this is an
+optimization, additive, and two-way (internal behavior, not wire shape —
+the report shape is unchanged).
+
+## Work
+
+When receive-pack implementation is on the critical path for a
+deployment with heavy stale-push traffic (measured, not hypothetical):
+add the pre-check to the transport push state machine, after command
+parse and before ingestion. Rejected refs report the same `ng <ref>
+<reason>`; the pack body is then drained (or the connection closed per
+the substrate's error rule).
+
+## Verification
+
+- All existing push tests still pass (real `git push` against the
+  transport).
+- A stale-old push errors before the pack body is fully read (observable
+  via the body-budget counters or a truncated-body test).
+
+## Out of scope
+
+- Changing the report shape or the protocol order (ADR-013 §7 stands).
+
+## Summary
+
+> Filled on completion.