feat(scheduler-sim-decision-trace): trace record model foundation

Task 1.3: trace types and constants, per-record arena, job selection
matching, no-decision record finishing; empty capture module; both
sources scanned by the core isolation test with a no-stdio check.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/scheduler/meson.build

@@ -35,6 +35,8 @@ scheduler_srcs = [
   'src/alloc_cache.c',
   'src/host_iter.c',
   'src/alloc_runner.c',
+  'src/sched_trace.c',
+  'src/sched_trace_capture.c',
   'src/plugins/fcfs.c',
   'src/plugins/builtin_fcfs.c',
   'src/plugins/seqjob_allocator.c',