refactor(scheduler-sim-decision-trace): split trace sources under the 800-line cap

Task 2.4: outcome capture moves to sched_trace_outcome.c and the
position/host/summary/rollback half of the model to sched_trace_scan.c;
shared helpers go behind sched_trace_internal.h (sti_ prefix). Code is
moved unchanged; every target that compiles the trace sources gets all
four, and the core isolation scan covers the new files.

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

src/scheduler/meson.build

@@ -36,7 +36,9 @@ scheduler_srcs = [
   'src/host_iter.c',
   'src/alloc_runner.c',
   'src/sched_trace.c',
+  'src/sched_trace_scan.c',
   'src/sched_trace_capture.c',
+  'src/sched_trace_outcome.c',
   'src/plugins/fcfs.c',
   'src/plugins/builtin_fcfs.c',
   'src/plugins/seqjob_allocator.c',