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/cluster.c

@@ -192,6 +192,17 @@ static int myreq_handler(vtask_t* task)
 
     init_reply_by_req(&reply, &request);
 
+    /* Dispatch gate: reject ops not allowed in current FSM state (req 1.3, 1.4) */
+    rc = ha_dispatch_gate(&c->ha, request.opcode);
+    if (rc != 0) {
+        reply.status     = rc;
+        reply.data_which = RequestReply_data_nodata;
+        xce_reply(ep, c->myhost, &reply);
+        free_Request(&request);
+        free(raw_pkt_copy);
+        return rc;
+    }
+
     switch (request.opcode) {
     case OPCode_shutdown:
         reply.status     = 0;