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/include/xce/errnox.h

@@ -21,7 +21,14 @@ enum {
     E_XCE_INTERNAL,
     E_XCE_NOMASTER,   /* Standby has no known primary — client should retry */
     E_XCE_REDIRECT,   /* Standby is redirecting client to primary           */
+    E_XCE_NOT_READY,  /* Controller in CTRL_INIT or CTRL_ELECTING state     */
+    E_XCE_STANDBY,    /* Controller in CTRL_STANDBY; op is primary-only     */
     E_XCE_MAX
 };
 
+/* HA dispatch gate error variables — defined in ha.c, declared here so all
+ * dispatch call sites that include errnox.h get the right names. */
+extern int XCE_ERR_NOT_READY;  /* == E_XCE_NOT_READY */
+extern int XCE_ERR_STANDBY;    /* == E_XCE_STANDBY   */
+
 #endif