feat(scheduler-core): implement record start/stop lifecycle (task 9.2)

Adds recording-state fields directly to xce_scheduler_t (no new
xce_sched_driver_t type, per task 9.1's precedent). sched_record_start()/
sched_record_stop() implement AC 10: validate via task 9.1's path
validator, open with a TOCTOU-hardened sequence (O_NOFOLLOW, no O_TRUNC,
fstat()+st_nlink==1 check before ftruncate()), snapshot the core, write
ReplayHeader+SnapshotEntry on start, ReplayEvent{endSession} on stop.

driver.c gains boolean-gated recording hooks in
xce_sched_driver_translate_pending_events() (per-event-batch ReplayEvent
writes, new recording_events.c) and xce_sched_driver_run_normal_mode_round()
(per-session ReplaySession writes, new recording_session.c) -- zero I/O/
allocation when inactive.

Two real bugs found and fixed during implementation: bug-1498 (O_TRUNC
would have truncated a hard-linked victim file before the TOCTOU check
could reject it) and bug-1499 (a transient edit dropped
xce_core_signal_go(), causing a 3-way thread deadlock).

test_record_lifecycle.c decodes the full start->events->session->stop
cycle with the real capnp decoders and asserts the exact message
sequence; TOCTOU regression tests use real symlink(2)/link(2) syscalls.

25/25 scheduler suite, clang-format clean.

src/scheduler/meson.build

@@ -12,6 +12,8 @@ scheduler_srcs = [
   'src/snapshot_load.c',
   'src/decision.c',
   'src/recording.c',
+  'src/recording_events.c',
+  'src/recording_session.c',
   'src/schedobj.c',
   'src/plugin.c',
   'src/resource.c',