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/testsuite/test_trace_model.c

@@ -32,7 +32,8 @@
  *     tallying hits per limit (5.9, 5.10);
  *   - consecutive released-host calls merge into one rollback entry (5.8).
  *
- * Links only src/sched_trace.c (plus libxce for the jobid helpers).
+ * Links only the model, src/sched_trace.c and src/sched_trace_scan.c (plus
+ * libxce for the jobid helpers).
  */
 #include <stdio.h>
 #include <stdlib.h>