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/meson.build

@@ -143,6 +143,7 @@ controller_unit_test_src = [
   'testsuite/test_drive_async_config.c',
   'testsuite/test_slowlane.c',
   'testsuite/test_commit_admission_pool.c',
+  'testsuite/test_ha_vep_teardown.c',
   'testsuite/test_waldump.c',
   'testsuite/waldump.c',
   'testsuite/test_walcompare.c',