feat(scheduler-sim-decision-trace): trace entry sequence and line budget

Task 1.4: requirement path push/pop with balanced begin/end entries
carrying the full nesting path, one entry per traced event kind, and a
line budget that stops storing at the bound and counts omitted lines.

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

src/scheduler/include/sched/alloc_iter.h

@@ -38,8 +38,12 @@
  * Produced by xce_alloc_plan_iter_t.next(); the returned pointer is valid
  * only until the following next() or fini() call on that same iterator
  * (it is not independently heap-owned by the caller).
+ *
+ * Tagged so sched/trace.h can forward-declare it without including this
+ * header (which pulls in scheduler.h -> core.h, and core.h will carry the
+ * trace record pointer).
  */
-typedef struct {
+typedef struct xce_alloc_plan_s {
     int nbuckets;      /* number of hosts this plan needs (==
                         * len(bucket_slots)) */
     int* bucket_slots; /* partition of maxTasks; bucket_slots[i] = slots