feat: implement real am_i_primary() with election probing, ha_is_primary(), ha_check_priority_stepdown()

- Replace stub am_i_primary() with real election probe logic
- probe_thread(): non-blocking TCP connect with optional SSL handshake
- wait_any_probe(): deadline-based spin-poll across all probe threads
- am_i_primary(): priority-0 always primary; higher priorities probe
  lower-priority peers and self-promote if deadline expires
- ha_is_primary(): lock-free role check
- ha_check_priority_stepdown(): simultaneous-boot stepdown at term=0
- start/stop_heartbeat_listener(): stubs for Task 7
- ha_init(): explicit initialization of ssl_ctx/jitter_seed/standby_endpoints
- TDD: add test_am_i_primary suite with 3 tests covering p0/p1 paths

src/controller/testsuite/controller_test.c

@@ -20,6 +20,7 @@ extern void test_e2e_hooks(void);
 extern void test_e2e_gc(void);
 extern void test_e2e_capnp(void);
 extern void test_ha_role(void);
+extern void test_am_i_primary(void);
 extern void test_ha_cluster(void);
 extern void test_ha_redirect(void);
 extern void test_p1_integration(void);
@@ -121,6 +122,7 @@ int main(int argc, char* argv[])
     TEST(test_e2e_gc);
     TEST(test_e2e_capnp);
     TEST(test_ha_role);
+    TEST(test_am_i_primary);
     TEST(test_ha_cluster);
     TEST(test_ha_redirect);
     TEST(test_p1_integration);