test(ha-commit-engine): RED integration tests for ACK feed and sync-append (task 5.1)

Tests 30-34: wired repl_on_ack feeds engine, membership mutators
trigger re-evaluation, per-entry sync-append contract (stale-ACK
rejection, committed_lsn unmoved on timeout, n_peers=1 replaces
-ENODEV). test_repl_majority walAck cases rewired to per-LSN
semantics (configAck byte-identical). Sanctioned scaffolding:
entry_lsn param on repl_wait_majority_ack (legacy body byte-identical,
callers pass 0), inert repl_set_commit_engine stub. 9 RED failures;
baseline signatures unchanged. LSN-0 edge documented for 5.2/5.3.

.kiro/specs/ha-commit-engine/tasks.md

@@ -53,7 +53,7 @@
   - _Requirements: 3.1, 3.2, 3.3, 6.1, 6.2_
 
 - [ ] 5. Integration: wire the engine behind the job-submit sync path
-- [ ] 5.1 Write RED integration tests for the ACK feed and the sync-append path
+- [x] 5.1 Write RED integration tests for the ACK feed and the sync-append path
   - Tests in `test_commit_engine.c`: `repl_on_ack` with a wired engine updates `acked_lsn` AND triggers commit evaluation (majority → `committed_lsn` advances, waiter wakes); `repl_start_catchup`/`repl_complete_catchup`/`repl_remove_standby`/`repl_add_standby` trigger re-evaluation; `persist_wal_sync_append` with wired engine returns 0 only on per-entry majority, returns `-ETIMEDOUT` when standbys' `acked_lsn` covers only an EARLIER entry (stale-ACK regression — connectivity/old ACKs never substitute), leaves `committed_lsn` unmoved on timeout, and for n_peers=1 returns 0 with `committed_lsn` advanced (replaces the old `-ENODEV` degraded path)
   - Update `testsuite/test_repl_majority.c` walAck cases to wire an engine and assert per-LSN semantics; configAck cases byte-identical
   - Observable completion: new/updated cases FAIL (red) against unwired production code
@@ -88,3 +88,7 @@
 - (2.1) WAIT/cap/gate tests (8-16) read `g_ce.inflight_waits` under `g_ce.lock` — task 2.2 must keep that field accurate while waiters block. Timing floors have ~20ms slack vs sleeps (reviewer advisory: watch for flake on loaded boxes). Unity assertions are main-thread-only after joins — keep that discipline in future threaded tests.
 - (3.1/3.2) Joint completion: task 1.2's commit_on_membership_change (commit.c:334-346) already implemented the full 3.2 contract, making 3.1's RED unachievable without dishonest tests. Tests 17-20 landed green as regression coverage; reviewer confirmed mutation-killing power (each test fails against the relevant mutant) and zero production changes. Lesson for later RED tasks: when a prior task's "minimal" allowance fully implements a later contract, complete the pair jointly under review rather than weakening tests.
 - (4.1) REVIEWER ADVISORY for 4.2: the gate-side fix (ha_state_t.repl back-pointer + catch-up exclusion through ha_has_write_quorum) has NO RED test — 4.1's tests would all pass even if 4.2 only implemented the helper. 4.2 MUST add a wired gate-level case (n_peers=3, catch-up-only standby → NOT_READY via ha_dispatch_gate) as its own green evidence. Tests 26-28 pin the NULL-back-pointer legacy fallback and n_peers<=1 contracts 4.2 must preserve. Stub repl_active_standby_count returns -1 (repl.c:557) — replace, don't wrap.
+- (5.1) **LSN-0 edge for 5.2/5.3 (reviewer-verified)**: commit_submit rejects lsn==0 with -EINVAL (commit.c:229), but the FIRST-EVER WAL entry is LSN 0 (current_lsn starts at 0; written_lsn = current_lsn - 1 at persist.c:1020). When 5.3 passes written_lsn, the first sync append on a fresh cluster would get -EINVAL. 5.3 must handle written_lsn==0 (options: treat entry 0 as valid in the engine, or shift LSN numbering, or special-case — decide per design). Tests 32-34 sidestep via a priming async append; production must not.
+- (5.1) persist.c currently passes entry_lsn=0 to the adapter — if 5.2 lands walAck delegation before 5.3 passes written_lsn, the wired sync path returns -EINVAL. Land 5.2's delegation gated on ce!=NULL AND entry_lsn!=0 falling back to legacy, OR land 5.2+5.3 wiring in lockstep.
+- (5.1) Baseline failure root causes identified: test_committed_lsn_does_not_advance_on_timeout (test_repl_unit.c:294) expects the OLD contract (rc==0 + sync_rc out-param) but persist_wal_sync_append now rejects-on-timeout — STALE TEST EXPECTATION, fix the test at 6.1, not the code. test_p2_integration.c:test_single_node_committed_lsn_unchanged pins the -ENODEV degraded path that 5.3 replaces — update it at 5.3/6.1.
+- (5.1) Repo hygiene (pre-existing, out of scope): untracked src/lib/testsuite/test_wal_wire.c is referenced by committed src/lib/meson.build:176 — should be added to git eventually.