Replace stub with full iomgr-native vote reply decoder:
- Reads RequestVoteReply fields into local scalars BEFORE capn_free (UAF fix)
- Term fencing: atomic update ha->term if peer term > ours
- Error path: decode failure counts peer as denied vote
- Test seam: xce_vote_reply_hook for unit testing without real packets
- 4 new ASAN-clean unit tests: field decode, denied vote, term fencing, no-UAF
The pre-existing vote_request_thread UAF in the old pthread path remains
but is targeted for removal in task 5.3 (old code removal).
- 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*
