feat(scheduler-core): implement simulation-mode startup (task 11.1)

Add sim.h/sim.c implementing AC 13 steps 1-4: open+validate ReplayHeader,
locate the most recent SnapshotEntry at/before --sim-start-ts, load it via
xce_sched_core_load_snapshot(), and set the virtual clock anchors. Wire
all seven simulation CLI flags into main.c's task-5.2 mode-selection
branch. Event loop itself (step 5) deferred to task 11.2. Fixes two real
bugs found during TDD: a SnapshotEntry_get_ts() use-after-free and a
pend_updates NULL-deref in xce_sched_core_fini() for bare-core callers.
27/27 scheduler suite green, full project build clean.

src/scheduler/meson.build

@@ -14,6 +14,7 @@ scheduler_srcs = [
   'src/recording.c',
   'src/recording_events.c',
   'src/recording_session.c',
+  'src/sim.c',
   'src/schedobj.c',
   'src/plugin.c',
   'src/resource.c',