feat(ha-convergence-recovery): ACK-driven catch-up continuation + parity re-inclusion (task 4.2)

src/controller/ha.h

@@ -349,6 +349,34 @@ int ha_rejoin_reconcile(cluster_t *c, uint64_t primary_committed_lsn);
 void cluster_apply_ctrlhello_reply(cluster_t *c, uint64_t committed_lsn,
                                    uint64_t config_tag);
 
+/*
+ * ha_walack_on_ack: the post-ACK catch-up decision (ha-convergence-recovery
+ * task 4.2, Req 5.1, 5.2, 7.1, design D6).  Called from the walAck branch of
+ * repl_ack_decode_and_dispatch (ha.c) AFTER repl_on_ack, and from the
+ * ha_walack_continuation_for_test shim so production and tests exercise the
+ * same logic.  Defined in config_repl.c (alongside ha_stream_wal_to_standby and
+ * the continuation task body).
+ *
+ *   target = max(committed_lsn, last_written)   (same derivation as ha.c:2771-2776)
+ *   if acked_lsn <  target → schedule ONE catch-up continuation that streams
+ *                            [acked_lsn+1, target] on delaymgr (never inline on
+ *                            iomgr), UNLESS c->ha.catchup_inflight[peer_idx] is
+ *                            already set (per-standby single-flight, Req 5.2).
+ *   if acked_lsn >= target → repl_complete_catchup(&c->persist.repl, ep) to
+ *                            re-include the standby in live replication (Req 7.1).
+ *
+ *   c        — cluster_t* (real persist + repl registry; never NULL).
+ *   ep       — repl registry endpoint key for the standby (endpoint_t* /
+ *              repl_ep_t); identifies the standby slot for streaming + parity.
+ *   peer_idx — 0-based standby slot index (into c->ha.catchup_inflight[]).
+ *   acked_lsn— the LSN the standby just acknowledged.
+ *
+ * Returns 0 on the parity / single-flight-suppressed paths and on a successfully
+ * scheduled continuation; negative on a scheduling/allocation failure.  Never
+ * performs file I/O itself (streaming runs in the deferred continuation task).
+ */
+int ha_walack_on_ack(cluster_t *c, void *ep, int peer_idx, uint64_t acked_lsn);
+
 
 /* ── Redirect helpers used by join.c ──────────────────────── */