feat: P2.5-1/2/2a/3/8 xce_cluster_config_t, xce_connect_cluster, xce_reconnect_step

Add libxce cluster discovery and auto-reconnect state machine:

- src/include/xce/cluster.h: xce_candidate_t, xce_cluster_config_t,
  xce_reconnect_ctx_t types; full API with test-seam hook
  xce_cluster_set_connect_fn()
- src/lib/cluster.c: xce_cluster_config_validate() (rejects invalid
  candidate counts, empty hostname/port, missing identity/ca_cert),
  xce_connect_cluster() (candidate fallback with single sweep),
  xce_reconnect_step() state machine (IDLE→CONNECTING→CONNECTED,
  with exponential backoff; nanosleep skipped when mock is active)
- src/lib/meson.build: add cluster.c to libxce_src and
  test_cluster.c to libxce_test_src (P2.5-2a)
- src/lib/testsuite/test_cluster.c: 9 TDD tests covering validation,
  first-candidate success, fallback, all-fail NULL return, reconnect
  to CONNECTED, and backoff state transition (P2.5-8)

All 9 cluster tests pass; 3 pre-existing unrelated failures unchanged.

src/lib/testsuite/test.c

@@ -32,6 +32,7 @@ extern void test_request(void);
 extern void test_zarray_cow(void);
 extern void test_kvmgr_capture(void);
 extern void test_hooks(void);
+extern void test_cluster(void);
 
 void setUp(void) { return; }
 
@@ -133,6 +134,7 @@ int main(int argc, char* argv[])
     TEST(test_zarray_cow);
     TEST(test_kvmgr_capture);
     TEST(test_hooks);
+    TEST(test_cluster);
 
     return UNITY_END();
 }