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

.kiro/specs/ha-config-bootstrap/tasks.md

@@ -124,7 +124,7 @@ Dependency order: startup validation (D) unblocks vtaskmgr before election (A);
   - _Requirements: 3.1, 1.3_
   - _Boundary: Config Bootstrap (config_repl.c)_
 
-- [ ] 5.3 Implement bootstrap_config_if_needed() and wire into startup
+- [x] 5.3 Implement bootstrap_config_if_needed() and wire into startup
   - Returns immediately if `config_tag != 0`
   - Probes all peers; three outcomes: all-zero → proceed; any non-zero → set `defer_config_fetch`, return; no peer reachable → retry with backoff 500ms→10ms→...→10s cap; log WARN each failed attempt
   - Integrate into `myapp.c`: call after `cluster_open_listener()` + `iomgr_init()` + `vtaskmgr_init()`, before `ha_wait_for_heartbeat_or_timeout()`