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>

.kiro/specs/scheduler-sim-decision-trace/tasks.md

@@ -32,7 +32,7 @@
   - Done when model unit tests pass under ASAN for selection matching, arena copying and size, and record finishing, and the isolation test passes with the new files scanned.
   - _Requirements: 1.2, 1.3, 1.8, 2.4, 9.1, 11.2_
 
-- [ ] 1.4 Build the trace entry sequence and the line budget, test first
+- [x] 1.4 Build the trace entry sequence and the line budget, test first
   - Path push and pop, with balanced requirement begin and end entries. Each entry stores its full nesting path: single, compound container, alternative i/n, or member j/m.
   - Entry kinds, each rendering as exactly one line:
     - requirement begin and end;
@@ -340,3 +340,4 @@
 - 1.3: `sched_trace_ctl_wants` whole-array selectors match elements only, not the bare key. A bare id in `trace on job <id>` can't be told apart from an array base for a job that doesn't exist yet, so 4.3's parser must emit both `{key,false}` and `{key,true}`, or the matcher must also accept `jobid == key`.
 - 1.3: design's record header lists a "file tag", but `xce_sched_trace_ctl_t` has no such field; the 4.2/4.6 store/emit tasks must set it or drop it explicitly.
 - 1.3: trace sources (sched_trace.c, sched_trace_capture.c) may not include <stdio.h> (core isolation test); capture copies raw strings/numbers and leaves formatting to the renderer.
+- 1.4: `sched_trace_host_verdict(rec, 0, err)` means "derive from rows" (rec->host_row_verdicts shortfall/limit, else SELECT, phase per-host); an explicit SELECT is the pre-filter phase. 2.2's row capture must set host_row_verdicts and 3.3/3.4 hooks must follow this. `sched_trace_pos_scan_error` stores no entry; it turns the next UNFILLED position end into SCAN_ERROR. Plan numbers restart per requirement. trace.h forward-declares `struct xce_alloc_plan_s` (tag added in alloc_iter.h) to avoid the core.h include loop. `resreq_begin` may store a NULL req name for a NULL rusage; 4.1's renderer must tolerate it.