test(ha-commit-engine): RED tests for per-LSN majority evaluation (task 1.1)

commit.h umbrella-verbatim interface + inert stub commit.c (test target
only); 7 Unity tests covering per-LSN vs acked_lsn>0 counting, majority
median, monotonicity, append clamp, lagging standby, RETAIN immediacy.
All 7 fail RED as intended. Documented 3 pre-existing masked suite
failures (bug-411) and the per-group exit-code masking root cause.

src/controller/commit.c

@@ -0,0 +1,68 @@
+/*
+ * commit.c — STUB for ha-commit-engine task 1.1 (RED phase).
+ *
+ * Behavioral functions are deliberately inert (-ENOSYS / no-op) so the
+ * task 1.1 tests in testsuite/test_commit_engine.c compile, link, and FAIL
+ * on their assertions. init/fini are real enough that tests exercise the
+ * engine's behavioral contract rather than failing at fixture setup.
+ *
+ * The real implementation lands in task 1.2 (per-LSN majority evaluation,
+ * monotonic/contiguous committed_lsn advancement).
+ */
+#include <errno.h>
+
+#include "commit.h"
+
+int commit_engine_init(commit_engine_t* ce, persist_t* p, int n_peers, int n_vcpus)
+{
+    if (ce == NULL || p == NULL || n_peers < 0 || n_vcpus < 1) {
+        return -EINVAL;
+    }
+    pthread_mutex_init(&ce->lock, NULL);
+    pthread_cond_init(&ce->cond, NULL);
+    ce->p              = p;
+    ce->n_peers        = n_peers;
+    ce->n_vcpus        = n_vcpus;
+    ce->inflight_waits = 0;
+    ce->writable_lsn   = 0;
+    return 0;
+}
+
+void commit_engine_fini(commit_engine_t* ce)
+{
+    if (ce == NULL) {
+        return;
+    }
+    pthread_cond_destroy(&ce->cond);
+    pthread_mutex_destroy(&ce->lock);
+    ce->p = NULL;
+}
+
+int commit_submit(commit_engine_t* ce, uint64_t lsn, commit_mode_t mode,
+                  uint32_t timeout_ms)
+{
+    (void)ce;
+    (void)lsn;
+    (void)mode;
+    (void)timeout_ms;
+    return -ENOSYS; /* stub: real WAIT/RETAIN semantics in task 1.2/2.2 */
+}
+
+void commit_on_ack(commit_engine_t* ce, int standby_idx, uint64_t acked_lsn)
+{
+    (void)ce;
+    (void)standby_idx;
+    (void)acked_lsn;
+    /* stub: never advances committed_lsn */
+}
+
+void commit_set_writable_lsn(commit_engine_t* ce, uint64_t noop_lsn)
+{
+    (void)ce;
+    (void)noop_lsn;
+}
+
+void commit_on_membership_change(commit_engine_t* ce)
+{
+    (void)ce;
+}