test(raft-drive-async): receiver-side dispatch liveness integration tests (task 3.4b)

With the dispatch flags flipped ON in-test, prove the three receiver-side
liveness properties (production defaults stay OFF):

- P1: while a snapshot (and config-push) transfer is parked in flight on the
  slow lane, another endpoint's peer-control message is still processed
  promptly on the vtask pool (different thread), with the transfer still
  parked — the offloaded transfer does not block the drive path.
- P2: a peer-control ack (classified VTASK_LIVENESS) runs promptly even while
  the pool is saturated to the LivenessAdmission reservation boundary with
  REQ_BLOCK tasks (one more REQ_BLOCK fast-rejects with -E_XCE_NOT_READY,
  proving real saturation). Narrowed to the liveness-dispatch-under-saturation
  assertion; commit-advance itself is covered by the commit-engine tests.
- P3: while one endpoint's peer-control body is parked (holding a worker + that
  ep busy), another endpoint's body runs concurrently on a different worker —
  per-endpoint FIFO does not serialize across endpoints.

All deterministic (cond-var gates, bounded polls). Fixed a flaky teardown-race
hang (bug-1224): the tests released a parked vtask body then immediately called
the cancel-based vtaskmgr_fini while the just-released body was still unwinding
(P2's REQ_BLOCK bodies racing blk_lock teardown the worst). Fix adds end-of-body
completion counters polled to the full expected count before fini, so every
pool body has fully returned (worker idle) before the cancel. slowlane_fini is
graceful (join) so the lane side was already safe.

Verified: 5/5 deterministic isolated runs; full controller suite EXIT=0;
ASan-clean. Completes task 3.4 (3.4a UAF fix + 3.4b tests); the production
flag-flip remains deferred to after the 4.1 partition soak + 4.2 regression
sweep.

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

src/controller/meson.build

@@ -144,6 +144,7 @@ controller_unit_test_src = [
   'testsuite/test_slowlane.c',
   'testsuite/test_commit_admission_pool.c',
   'testsuite/test_ha_vep_teardown.c',
+  'testsuite/test_ha_dispatch_integration.c',
   'testsuite/test_waldump.c',
   'testsuite/waldump.c',
   'testsuite/test_walcompare.c',