fix(controller): offload checkpoint_task_fn off the delaymgr thread (bug-1694)

Last of the 9 confirmed findings from the /xce-audit-invariants controller
pass. checkpoint_task_fn was registered directly as a delaymgr entry and
ran its full blocking body inline: checkpoint marker pwrite+fdatasync,
write_snapshot()'s genuinely unbounded per-entity walk (scales with
cluster state size) held under apply_pump_lock, WAL trim, and snapshot
rotation/unlink -- all on the same thread that drives heartbeats/election
timeouts. Unlike every other delaymgr operation (snapshot send,
config-push, reconnect, catch-up), checkpoint had no offload mechanism
at all -- the one gap left from the raft-drive-async hardening pass.

Design: full offload via the SlowLane (slowlane_submit), default ON --
NOT the vtask pool. persist_apply_pump_run() already holds apply_pump_lock
across its own per-entry walk while running ON the vtask pool
(VTASK_LIVE_BLOCK); putting checkpoint's unbounded, lock-held
write_snapshot() on that same pool risked starving it, exactly what
ha-async-io.md's own thread-role table warns against. The SlowLane is
the actual established pattern for this I/O class (push_config_transport_only,
the snapshot-send path, catchup_lane_fn, demotion_lane_fn all dispatch
there) -- and persist_checkpoint() already runs on the slow lane in
production TODAY via ha_snapshot_do_send() -> snapshot_lane_fn, so
apply_pump_lock's acquisition from a slow-lane thread is proven-safe,
exercised daily, not new surface. Unlike catchup/demotion (which stay
flag-gated OFF for an unrelated, still-not-root-caused hang, bug-1429),
checkpoint's new flag defaults ON given this reuses an already-proven
code path. Inline-fallback + WARN only on a genuine slowlane_submit
failure (not -EAGAIN/-EEXIST, both non-failures) -- never silently drops
a scheduled checkpoint.

New test_persist_checkpoint_offload.c (3 tests). RED-phase forced the
pre-fix always-inline path: the suite never printed a single result and
had to be killed, proving the test genuinely detects the violation, not
a tautology. GREEN: all 3 pass. Full suite: 1184 tests, 2 failures --
both the pre-existing, unrelated bug-1561 overlayfs sandbox flake
(exact signature match). No live HA soak was run for this specific fix
in isolation (explicitly scoped out, not hidden) -- the mandatory
live-soak proof for this whole audit campaign covers it next.

This closes the last of the 9 confirmed findings from the controller
audit (bug-1684, bug-1687/1689/1690/1691/1692, bug-1693, bug-1694).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AgYRU3zDTqNGV5Jk8aiKLg

src/controller/meson.build

@@ -180,6 +180,7 @@ controller_unit_test_src = [
   'testsuite/test_apply_correctness_verification.c',
   'testsuite/test_apply_pump.c',
   'testsuite/test_apply_pump_integration.c',
+  'testsuite/test_persist_checkpoint_offload.c',
   'testsuite/test_job_array_apply.c',
   'testsuite/test_job_array_fence.c',
   'testsuite/test_job_array_replay_fence.c',