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/meson.build

@@ -112,6 +112,7 @@ controller_unit_test_src = [
   'testsuite/test_ha_ctrlhello.c',
   'testsuite/test_ha_failover.c',
   'testsuite/test_ha_vote_send.c',
+  'testsuite/test_ha_start_election_async.c',
   'startup_validation.c',
   'worker.c',
   'persist.c',