feat(election-iomgr-refactor): add vote_ep_ctx_t and on_vote_connected

- vote_ep_ctx_t: per-peer context (ha, c, ctx, peer_idx, term/tag/lsn/name)
- election_timeout_ctx_t: cancelled flag + context for delay watchdog
- on_vote_connected(): iomgr callback fires after SSL handshake; builds
  xce_request_t{opcode=OPCode_requestVote}, calls xce_request, sets msg_handler
- vote_reply_msg_handler stub (returns 0; implemented in task 2.3)
- Test seam: xce_vote_request_hook for unit testing without live SSL
- 4 new unit tests: opcode, field encoding, msg_handler set, cancelled flag

Context note: vctx->c is NULL in test helper — task 2.3 must use real cluster_t*

src/controller/testsuite/controller_test.c

@@ -43,6 +43,7 @@ extern void test_persist_truncate(void);
 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);
 
 void setUp(void) { return; }
 
@@ -155,6 +156,7 @@ int main(int argc, char* argv[])
     TEST(test_ha_connect);
     TEST(test_ha_ctrlhello);
     TEST(test_ha_failover);
+    TEST(test_ha_vote_send);
 
     return UNITY_END();
 }