feat(ha-repl-hardening): add observability log statements (task 5.4)

- config_repl.c: INFO at WAL catch-up start with start/target LSN (Req 7.1)
- config_repl.c: INFO at snapshot transfer start and complete with
  checkpoint_lsn and size (Req 7.3)
- ha.c committing_timeout_fn_for_test: add WARN log counting
  n_converged/n_pending before promotion, matching production path (Req 7.5)
- Reqs 7.2 (standby converged INFO) and 7.4 (majority INFO) were
  already present from tasks 3.1 and 5.2 respectively

src/controller/config_repl.c

@@ -1441,6 +1441,11 @@ int ha_stream_wal_to_standby(endpoint_t *ep, const char *wal_path,
     if (standby_lsn >= committed_lsn)
         return 0; /* nothing to stream */
 
+    /* Req 7.1: log WAL catch-up start with start/target LSN */
+    INFO("ha_stream_wal_to_standby: starting WAL catch-up "
+         "start_lsn=%" PRIu64 " target_lsn=%" PRIu64,
+         standby_lsn + 1, committed_lsn);
+
     /* ── Enumerate WAL segment files, sort ascending by first LSN ── */
     DIR *d = opendir(wal_path);
     if (!d)
@@ -1586,6 +1591,11 @@ int ha_send_snapshot_to_standby(endpoint_t *ep, const char *snap_dir,
         return -errno;
     uint64_t total_size = (uint64_t)st.st_size;
 
+    /* Req 7.3: log snapshot transfer start with checkpoint_lsn and size */
+    INFO("ha_send_snapshot_to_standby: snapshot transfer start "
+         "checkpoint_lsn=%" PRIu64 " size=%" PRIu64,
+         checkpoint_lsn, total_size);
+
     FILE *f = fopen(snap_path, "rb");
     if (!f)
         return -errno;
@@ -1647,6 +1657,13 @@ int ha_send_snapshot_to_standby(endpoint_t *ep, const char *snap_dir,
 
     free(buf);
     fclose(f);
+
+    /* Req 7.3: log snapshot transfer complete (on success only) */
+    if (rc == 0)
+        INFO("ha_send_snapshot_to_standby: snapshot transfer complete "
+             "checkpoint_lsn=%" PRIu64 " size=%" PRIu64,
+             checkpoint_lsn, total_size);
+
     return rc;
 }