feat(election-iomgr-refactor): implement ha_start_election_async — task 2.4

Non-blocking election initiator that returns immediately after registering
vote endpoints with iomgr and scheduling the election timeout watchdog.

- Single-node shortcircuit (n_peers==1): allocates self-only ha_elect_ctx_t,
  calls ha_elect_evaluate_and_signal directly, frees ctx inline — no iomgr
  or delaymgr involvement; returns in < 1ms
- Multi-node: allocates ha_elect_ctx_t, per-peer vote_ep_ctx_t, calls
  endpoint_connect + iomgr_monitor for each peer; schedules election timeout
  via delay_task_ms; skips peer on connect failure (counted as denied)
- election_timeout_fn: delaymgr callback that fires ha_elect_evaluate_and_signal
  after election_timeout_ms, unless cancelled flag is set (lock-free cancellation)
- vote_ep_err_handler: counts unreachable peers as denied votes
- Declares ha_start_election_async in ha.h with full doc comment
- 5 Unity tests in test_ha_start_election_async.c: returns < 5ms, stores
  n_peers, shortcircuit fires immediately, n_peers stored for multi-peer,
  term incremented — all GREEN with ASAN enabled

src/controller/testsuite/controller_test.c

@@ -44,6 +44,7 @@ extern void test_ha_connect(void);
 extern void test_ha_ctrlhello(void);
 extern void test_ha_failover(void);
 extern void test_ha_vote_send(void);
+extern void test_ha_start_election_async(void);
 
 void setUp(void) { return; }
 
@@ -157,6 +158,7 @@ int main(int argc, char* argv[])
     TEST(test_ha_ctrlhello);
     TEST(test_ha_failover);
     TEST(test_ha_vote_send);
+    TEST(test_ha_start_election_async);
 
     return UNITY_END();
 }