feat(ha-commit-barrier-rollout): persist_commit_barrier + legacy walAck retirement (task 2.2)

Barrier: watermark = current_lsn - 1 under p->lock (released before
blocking), covered/nothing-appended/LSN-0 short-circuits, sync_timeout_ms
fallback, engine code unmodified, ce==NULL vacuous. repl.c walAck track
now engine-only (legacy acked_lsn>0 counting and walAck-reachable
n_active==0 success deleted); configAck track byte-equivalent for
task 5. 15/15 group; census 472/472.

.wolf/cerebrum.md

@@ -9,6 +9,7 @@
 <!-- How the user likes things done. Code style, tools, patterns, communication. -->
 
 ## Key Learnings
+- repl_wait_majority_ack walAck track is engine-only since barrier-rollout 2.2: entry_lsn==0 returns 0 immediately (documented once-per-lifetime LSN-0 first entry), everything else delegates to commit_submit unconditionally — ce==NULL surfaces as -EINVAL from commit_submit (unreachable in production; cluster_init_phase2 always wires the engine). The counting loop that remains in the function is configAck-ONLY (no self-count, n_active==0 shortcut is Req 8.5 config behavior). persist_wal_sync_append's residual non-engine branch can only store committed_lsn for the LSN-0 no-op case. (2026-06-12, task 2.2)
 - `commit_submit(COMMIT_MODE_RETAIN)` is fully lock-free: it returns 0 BEFORE `pthread_mutex_lock(&ce->lock)` (commit.c:233-239), so it is safe to call from hook functions even while entity write locks are held — only WAIT mode takes ce->lock. (barrier-rollout task 1.2, 2026-06-12)
 - ha-commit-engine: once repl.c references commit_* symbols (task 5.2), commit.c MUST be in controller_src (meson.build) or the production xce_controller link fails — the "linkage lands with 5.3" note in tasks.md only held while repl.c had no engine references. Lock order ce->lock → r->lock: repl.c calls engine hooks only AFTER releasing r->lock.