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/ha.h

@@ -442,6 +442,31 @@ void ha_on_heartbeat_received(ha_state_t *ha);
 /* ha_disarm_heartbeat_timeout_vtask: stop the heartbeat watchdog thread. */
 void ha_disarm_heartbeat_timeout_vtask(ha_state_t *ha);
 
+/* ── Task 2.4: Non-blocking async election initiator ────────────────────── */
+
+/*
+ * ha_start_election_async: initiate a quorum election without blocking.
+ *
+ * Steps:
+ *   1. Store n_peers in ha->n_peers.
+ *   2. For single-node (n_peers == 1): call ha_elect_evaluate_and_signal directly
+ *      with a self-only ha_elect_ctx_t — shortcircuit, no iomgr/delaymgr needed.
+ *   3. For multi-node: for each peer in peers[], allocate vote_ep_ctx_t, init
+ *      endpoint with client_ssl_ctx, set on_vote_connected + vote_ep_err_handler,
+ *      call endpoint_connect + iomgr_monitor; skip peer on connect failure.
+ *      Allocate election_timeout_ctx_t (cancelled=0) and schedule
+ *      delay_task_ms(delaymgr, election_timeout_ms, 1, election_timeout_fn, ctx).
+ *   4. Returns immediately; election result delivered via ha_elect_evaluate_and_signal.
+ *
+ * Observable: returns in < 5ms; endpoints registered in iomgr; watchdog in delaymgr.
+ * Requirements: 1.3 (non-blocking), 2.1 (single-node shortcircuit), 2.4 (async vote)
+ * Boundary: ha.c (election orchestration)
+ */
+void ha_start_election_async(ha_state_t *ha, cluster_t *c,
+                             int n_peers, xce_candidate_t **peers,
+                             uint64_t my_config_tag, uint64_t my_lsn,
+                             const char *my_name);
+
 /* ── Task 9.2: Persistent Control Channel (Req 6.1) ───────────────────────── */
 
 /*