feat(ha-config-bootstrap): implement dispatch gate (task 3.3)

- errnox.h: add E_XCE_NOT_READY and E_XCE_STANDBY error codes with
  extern int XCE_ERR_NOT_READY / XCE_ERR_STANDBY runtime variables
- ha.h: declare ha_dispatch_gate(ha_state_t *, int opcode)
- ha.c: define XCE_ERR_* variables; initialize heartbeat_seen semaphore
  and primary_ctrl_fd = -1 in ha_init(); sem_destroy in ha_fini();
  implement ha_dispatch_gate() with allowlist for CTRL_INIT/CTRL_ELECTING
  (only OP_requestVote and OP_probeConfigTag pass) and CTRL_STANDBY
  (replication, election, and housekeeping ops pass; primary-only ops
  return -XCE_ERR_STANDBY)
- cluster.c: wire ha_dispatch_gate() in myreq_handler() before opcode
  switch; gate failure sends error reply and returns early

All 393 unit tests pass. Satisfies requirements 1.3 and 1.4.

src/controller/ha.h

@@ -277,6 +277,17 @@ void start_heartbeat_listener(cluster_t *c);
  */
 void stop_heartbeat_listener(cluster_t *c);
 
+/*
+ * ha_dispatch_gate: role-based operation filter for the vtaskmgr dispatch loop.
+ * Returns:
+ *   0                   — operation is permitted for the current role
+ *   -E_XCE_NOT_READY    — role is CTRL_INIT or CTRL_ELECTING and opcode is not
+ *                         OP_requestVote or OP_probeConfigTag
+ *   -E_XCE_STANDBY      — role is CTRL_STANDBY and opcode is a primary-only op
+ * CTRL_PRIMARY always returns 0.
+ */
+int ha_dispatch_gate(ha_state_t *ha, int opcode);
+
 /* ── HA Config Bootstrap & Term Durability (ha-config-bootstrap spec) ── */
 
 /*