test(ha-commit-barrier-rollout): RED tests for persist_commit_barrier (task 2.1)

7 cases: two covered short-circuits, timeout with committed_lsn
unmoved, covering-ACK release, sync_timeout_ms fallback (bounded both
directions), n_peers=1 standalone, NOT_READY passthrough via cap
saturation. Watermark resolved to current_lsn - 1 (design's literal
phrasing unsatisfiable per persist.h:71 invariant) with off-by-one
killing power both ways. 15 tests / 7 RED; suite baseline zero.

src/controller/persist.c

@@ -1171,6 +1171,21 @@ wal_commit_mode_t wal_classify(uint32_t wal_type, int detail)
     }
 }
 
+/* Commit barrier helper — INERT RED stub (ha-commit-barrier-rollout task
+ * 2.1; implemented in task 2.2). Returns a wrong sentinel so the task-2.1
+ * tests fail on assertions, not linking. Task-2.2 contract (design.md
+ * §Commit Barrier Helper): snapshot the appended watermark (current_lsn - 1
+ * under p->lock), return 0 when committed_lsn already covers it (or nothing
+ * appended), else commit_submit(WAIT, timeout_ms ? timeout_ms :
+ * p->config.sync_timeout_ms) with the engine code passed through
+ * unmodified. */
+int persist_commit_barrier(persist_t* p, uint32_t timeout_ms)
+{
+    (void)p;
+    (void)timeout_ms;
+    return -ENOSYS;
+}
+
 /* Classified append at the hook seam (barrier-rollout task 1.2).
  *
  * WAIT (queue/worker control): the append stays async here — the handler