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_driver_session_ordering.c

@@ -229,6 +229,24 @@ int sched_record_write_events(xce_scheduler_t* sched, xce_core_event_t* events,
     return 0;
 }
 
+/* sched_record_start/sched_record_stop (scheduler-core spec, task 9.4) --
+ * driver.c's handle_deferred_admin_record() (new this task) calls both;
+ * this file never enqueues an XCE_SCHED_OP_ADMIN_RECORD request, so
+ * neither is actually invoked at runtime, same rationale as
+ * sched_record_write_events above. */
+int sched_record_start(xce_scheduler_t* sched, const char* record_file)
+{
+    (void)sched;
+    (void)record_file;
+    return 0;
+}
+
+int sched_record_stop(xce_scheduler_t* sched)
+{
+    (void)sched;
+    return 0;
+}
+
 sched_record_session_ctx_t* sched_record_session_begin(xce_scheduler_t* sched,
                                                        uint64_t begin_round_ts)
 {