fix(raft-drive-async): endpoint-quiescence for bug-1197 vote-ep teardown (task 3.4a)

When the (still-default-OFF) peer-control dispatch flag is ON,
raft_cb_become_follower runs as a vtask pool worker and frees every peer
vote-ep while other pool workers may still hold those eps' data — a UAF
(bug-1197). This makes that path quiescence-safe so the flag can later be
flipped ON (deferred to the 4.x prod-flip gate; production stays OFF here).

Mechanism (no libxce-wide endpoint refcount; reuses the existing per-ep FIFO +
busy serialization):

- Deferred teardown-as-vtask: the loser loop, when dispatch is ON and the pool
  is wired, does per vote-ep endpoint_mark_tearing_down -> iomgr_unmonitor ->
  enqueue a VTASK_IO/VTASK_LIVENESS teardown vtask bound to that ep. Per-ep busy
  FIFO makes it run strictly after any in-flight/queued same-ep peer-control
  vtask; the body NULLs iod->ep before endpoint_fini+free(ep) to disarm
  run_task's post-body endpoint_unblock_tasks (which would touch the freed
  ep->lock). OFF / pool-unwired path is byte-for-byte the legacy endpoint_fini.

- RCU-like drain closes the dispatch/teardown TOCTOU window: new _Atomic int
  endpoint_t.dispatch_inflight (+ tearing_down:1 bit). The ON dispatch brackets
  its flag-check+enqueue with fetch_add/fetch_sub (seq_cst);
  endpoint_mark_tearing_down sets the flag under ep->lock, releases, then spins
  until dispatch_inflight drains to 0. So every dispatch that read the flag
  false finishes its enqueue before mark returns (FIFO-before the teardown
  vtask); any later dispatch reads the flag set and drops the packet. ep->lock
  is deliberately NOT held across the enqueue (it inverts with the taskq lock in
  vtask_eligible); the counter is the lock-free handshake. Drain is bounded: the
  held window is just a calloc + non-blocking enqueue on the single io_worker.

Still open before the production flag flip (4.x): the borrowed-ep race in
snapshot_lane_fn/configpush_lane_fn is distinct (slowlane thread vs io_worker
RDHUP teardown, not the vtask-pool FIFO) and needs its own per-standby-ep
discipline.

Tests: test_ha_vep_teardown.c — 5 ASan tests (ordering, late-event drop, run_task
unblock disarm, OFF-path unchanged, deterministic mark-drains-inflight-dispatch).
xcelib_test 282/0; controller_unit_test EXIT=0; ASan-clean. Adversarial
cpp-review: no CRITICAL/HIGH; handshake formally sound, flag-ON path race-free.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S6fDCtXkkEgupu7rUah1y2

src/controller/ha.h

@@ -1313,6 +1313,27 @@ int  ha_peerctrl_dispatch_enabled(void);
  * production (zero cost) — a plain global like ha_nop_req_hook. */
 extern void (*ha_peerctrl_body_probe)(endpoint_t *ep);
 
+/*
+ * ha_vep_teardown_probe / ha_vote_ep_teardown_for_test (bug-1197): test seams
+ * for the deferred vote-ep teardown.  ha_vep_teardown_probe (NULL in production)
+ * fires with the real ep at the top of the teardown vtask body, before
+ * endpoint_fini, so a test can assert teardown ran strictly after the last live
+ * same-ep use.  ha_vote_ep_teardown_for_test drives the flag-ON per-ep async
+ * teardown branch (mark + iomgr_unmonitor + enqueue) for a single ep without
+ * staging a full election.  Both compiled only under XCE_TESTING.
+ */
+extern void (*ha_vep_teardown_probe)(endpoint_t *ep);
+void ha_vote_ep_teardown_for_test(ha_state_t *ha, endpoint_t *ep);
+
+/*
+ * ha_peerctrl_predispatch_probe (bug-1197): test seam fired inside the dispatch
+ * path AFTER the tearing_down check passes but BEFORE the pool enqueue (within
+ * the ep->dispatch_inflight bracket).  A test installs it to park a dispatch in
+ * exactly the drained window and prove endpoint_mark_tearing_down blocks until
+ * the enqueue completes.  NULL in production; compiled only under XCE_TESTING.
+ */
+extern void (*ha_peerctrl_predispatch_probe)(endpoint_t *ep);
+
 /* ── Task 2.4: Non-blocking async election initiator ────────────────────── */
 
 /*