feat(ha-config-bootstrap): implement bootstrap_config_if_needed() (task 5.3)

- Probe all peers: any tag>0 -> defer_config_fetch=true; all-zero -> proceed;
  none reachable -> exponential backoff 500ms->10s cap with WARN per attempt
- Wire into myapp.c: before ha_wait_for_heartbeat_or_timeout()
- Post-election: defer_config_fetch && CTRL_STANDBY -> fetch_config_from_primary()
- INFO log for all three bootstrap outcomes (Req 10.2)
- XCE_TESTING hook for probe mock; test_ha_bootstrap_support.c installs mock
- 9/9 bootstrap tests GREEN, 141/141 full suite GREEN

src/controller/config_repl.h

@@ -133,6 +133,17 @@ char *sanitise_cn(const char *src, size_t maxlen);
 #ifdef XCE_TESTING
 #include <pthread.h>
 #include <stdatomic.h>
+#include <stdint.h>
+
+/*
+ * xce_probe_config_tag_hook: test-only injection point for bootstrap probing.
+ * When non-NULL, bootstrap_config_if_needed() calls this instead of the real
+ * xce_probe_config_tag(), allowing unit tests to return controlled tag values
+ * without real network connections.
+ * Set by test infrastructure (e.g. test_ha_bootstrap_support.c) before tests run.
+ */
+extern int (*xce_probe_config_tag_hook)(const char *host, uint16_t port,
+                                        uint64_t *tag_out);
 
 /*
  * load_primary_config_for_test: test-only wrapper that exercises the path