feat(ha-commit-engine): engine instantiation + sync-append path switch (task 5.3)

cluster_t embeds commit_engine_t; init in cluster_init_phase2 after the
vtaskmgr guard (idempotence-guarded — phase2 can run twice), wiring
repl_set_commit_engine + ha.repl back-pointer before any standby can
register; fini after iomgr/vtask shutdown. persist_wal_sync_append
passes written_lsn; engine-wired path drops -ENODEV and the
committed_lsn store (engine owns advancement); LSN-0 first-entry
routes to legacy per the accepted interim. p2 -ENODEV pin updated to
the Req 6.3 contract. 34/34 + 6/6 green through honest delegation;
census exactly at the 3 validated baselines; production binary links.

src/include/xce/cluster_def.h

@@ -18,6 +18,7 @@
 #include "vtaskmgr.h"
 
 /* ha.h forward-declares cluster_t; including it here is safe (no circular dep) */
+#include "commit.h"
 #include "ha.h"
 #include "xce/types.h"
 #include "xlist.h"
@@ -121,6 +122,7 @@ typedef struct cluster_s {
     int                 running;
     persist_t           persist;   /* Persistence module — P0-F */
     ha_state_t          ha;        /* HA role management — P1-5 */
+    commit_engine_t     commit;    /* Per-entry commit engine — ha-commit-engine 5.3 */
     _Atomic int         restart_requested;           /* set by OP_upgradeConfig handler */
     pthread_t           heartbeat_listener_tid;      /* pre-Phase-2 SSL listener thread */
     _Atomic int         heartbeat_listener_running;