feat(scheduler-core): implement scheduler-side admin request handler (task 9.4)

Implements the scheduler's first request-reply opcode (OPCode_schedAdmin).
sched_handle_admin_request() (handler.c) decodes SchedAdminRequest and
dispatches by action: recordStatus answers synchronously (pure read);
recordStart/recordStop are deferred onto the existing pend_updates queue
(XCE_SCHED_OP_ADMIN_RECORD) since sched_record_start()'s snapshot call
requires the session_active==0 precondition only guaranteed on the driver
thread, not the async request-handling thread. driver.c's
xce_sched_driver_translate_pending_events() performs the deferred action
and sends the reply via a new xcecli_reply_schedadmin() (client.c/client.h),
correlated by seq copied before the original request is freed.

Adversarial review found a genuine cross-thread race: sched_record_status()
read driver-owned recording state (including a pointer the driver thread
frees) from the vtask thread with no lock -- the codebase's first such
race, missed by ASan (doesn't detect races). Fixed with a dedicated
record_lock scoped strictly to in-memory field access, never spanning file
I/O; sched_record_status() now strdup()s under the lock instead of
returning a borrowed live pointer. Verified with a real two-thread
lock-contention test (independently confirmed non-vacuous by removing the
lock and observing the test fail) plus a deterministic use-after-free
reproduction.

26/26 scheduler suite, clang-format clean.

src/scheduler/testsuite/test_record_lifecycle.c

@@ -342,6 +342,15 @@ static void fixture_init(xce_scheduler_t* sched, char* record_dir)
 
     memset(sched, 0, sizeof(*sched));
 
+    /* record_lock (scheduler.h field comments, task 9.4 fix round) is
+     * normally initialized by sched_init(), which this white-box fixture
+     * deliberately bypasses (same rationale as this function's own header
+     * comment) -- initialize it here too, same pairing as the
+     * xce_sched_core_init() call just below initializing core.lock. Every
+     * sched_record_start()/sched_record_stop() call in this file now takes
+     * this lock. */
+    mylock_init(&(sched->record_lock));
+
     rc = xce_sched_core_init(&(sched->core));
     TEST_ASSERT_EQUAL_INT(0, rc);