feat: P2-4 wire repl_broadcast into persist_wal_async/sync_append; last_replicated_crc

- Move repl.h from controller/ to include/ (shared header) to break
  persist.h ↔ repl.h circular include
- Extract wal_entry_hdr_t + wal_entry_type into include/wal_types.h so
  repl.h can include only what it needs without pulling in all of persist.h
- Add replicator_t repl field to persist_t (embedded by value, P2-4)
- Wire repl_init/repl_fini into persist_init/persist_fini lifecycle
- Wire repl_broadcast into persist_wal_async after each WAL write;
  last_replicated_crc updated here (not in persist_write_checkpoint_marker)
- Wire repl_wait_ack into persist_wal_sync_append; returns 0 on local
  WAL success; sync_rc_out carries -ENODEV/-ETIMEDOUT for degraded mode
- Add 4 TDD tests in test_persist_repl.c: structural repl field check,
  async no-standbys succeeds, sync degraded mode rc, checkpoint CRC invariant

src/include/repl.h

@@ -0,0 +1,60 @@
+#pragma once
+/*
+ * repl.h — P2-1: Replicator structures and public API.
+ * Fan-out TCP replication via existing iomgr/SSL endpoint pipeline.
+ * ACKs processed in io_worker read callback (NOT vtaskmgr) to prevent deadlock.
+ */
+#include <pthread.h>
+#include <stdatomic.h>
+#include <stdint.h>
+#include <time.h>
+#include "config.h"
+#include "mylock.h"
+#include "wal_types.h"  /* wal_entry_hdr_t — P2-4: use wal_types.h to break circular include */
+
+/* Endpoint is an opaque pointer — full type in netio.h (avoid heavy include) */
+typedef void* repl_ep_t;
+
+/* Per-standby connection slot */
+typedef struct {
+    repl_ep_t          ep;              /* NULL = slot empty */
+    _Atomic uint64_t   acked_lsn;      /* Last LSN ACKed by this standby */
+    _Atomic int        in_catch_up;    /* Non-zero while catching up */
+    uint64_t           join_applied_lsn; /* LSN standby reported on reconnect */
+    char               host[XCE_MAXNAME_LEN];
+    uint32_t           port;
+    uint64_t           last_disconnected_lsn;
+    time_t             last_disconnect_time;
+} standby_conn_t;
+
+/* Replicator — embedded in persist_t or cluster_t (P2-4).
+ * Named tag replicator_s matches the forward declaration in persist.h. */
+typedef struct replicator_s {
+    standby_conn_t  standbys[MAX_STANDBYS];
+    int             nstandbys;          /* Active standby count */
+    mylock_t        lock;               /* Protects standbys[], nstandbys */
+    pthread_cond_t  ack_cond;           /* Signaled when any ACK arrives */
+    pthread_mutex_t ack_lock;           /* Guards ack_cond */
+} replicator_t;
+
+/* Lifecycle */
+int  repl_init(replicator_t *r);
+void repl_fini(replicator_t *r);
+
+/* Standby management */
+int  repl_add_standby(replicator_t *r, repl_ep_t ep, const char *host, uint32_t port);
+void repl_remove_standby(replicator_t *r, repl_ep_t ep);
+void repl_expire_disconnect_slots(replicator_t *r, time_t now_s, uint32_t snap_interval_s);
+
+/* Replication send (stub in P2-1; wired in P2-4) */
+int  repl_broadcast(replicator_t *r, const wal_entry_hdr_t *hdr,
+                    const void *payload, uint32_t payload_len);
+
+/* ACK handling — call from io_worker read callback ONLY */
+void repl_on_ack(replicator_t *r, repl_ep_t ep, uint64_t acked_lsn);
+
+/* Sync replication (stub in P2-1; wired in P2-3) */
+int  repl_wait_ack(replicator_t *r, uint64_t target_lsn, uint32_t timeout_ms);
+
+/* WAL trim guard */
+uint64_t repl_trim_lsn(replicator_t *r, uint64_t checkpoint_lsn, uint32_t snap_interval_s);