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/testsuite/test_ha_bootstrap_support.c

@@ -0,0 +1,41 @@
+/* test_ha_bootstrap_support.c — Test fixture for bootstrap probe injection.
+ *
+ * This file is compiled into the controller_unit_test binary under XCE_TESTING.
+ * It installs a mock probe function via __attribute__((constructor)) before any
+ * test runs, so that test_ha_bootstrap.c tests do not need real network peers.
+ *
+ * Mock probe behaviour (keyed by port):
+ *   port 59901 → tag = 0xDEAD0001  (non-zero: peer has a config)
+ *   port 59902 → tag = 0           (zero: peer has no config)
+ *   port 59900 → tag = 0           (zero: peer has no config)
+ *   any other  → tag = 0           (zero: treat as unconfigured)
+ *
+ * Requirements: 3.2 (any peer tag>0 → defer), 3.3 (all-zero → no defer)
+ */
+#ifdef XCE_TESTING
+
+#include <stdint.h>
+
+#include "config_repl.h"
+
+static int mock_xce_probe_config_tag(const char *host, uint16_t port,
+                                     uint64_t *tag_out)
+{
+    (void)host;
+    /* Port 59901: simulate a peer that already has a config (non-zero tag) */
+    if (port == 59901) {
+        *tag_out = 0xDEAD000100000001ULL;
+        return 0;
+    }
+    /* All other test ports return tag=0 (unconfigured peer) */
+    *tag_out = 0;
+    return 0;
+}
+
+__attribute__((constructor))
+static void install_bootstrap_probe_mock(void)
+{
+    xce_probe_config_tag_hook = mock_xce_probe_config_tag;
+}
+
+#endif /* XCE_TESTING */