feat(scheduler-sim-decision-trace): wire the trace commands into the REPL

Task 5.2: trace is routed with the raw line and info trace through info;
help gains a Trace section; set output rejects open trace files; quit
and EOF shut tracing down; the continue-mode rollup reports a traced
round on its own; repl.h gains the interactive-only trace gates. The
baseline transcript changes only in the help and info-usage lines.

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

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

@@ -276,7 +276,7 @@
   - Done when all of the following pass under ASAN: a full scheduler build including the scheduler binary, the full scheduler test suite, and the simulation isolation test.
   - _Requirements: 10.5, 11.3_
 
-- [ ] 5.2 Wire the trace commands into the REPL
+- [x] 5.2 Wire the trace commands into the REPL
   - Route `trace` with the raw line, and route `info trace`.
   - Help gains a trailing Trace section, and the info subcommand list and info usage line include `info trace`.
   - `set output` rejects an open trace file. Quit and end of input shut tracing down.
@@ -371,3 +371,4 @@
 - 4.5: the trace FILE* and its dev/ino belong to the refcounted settings, and the last unref flushes and closes it. New functions in repl_internal.h: `repl_trace_settings_file()`, `repl_trace_file_conflicts(sim, path)` (5.2 wires it into `set output`) and `repl_trace_shutdown(sim)` (5.2 calls it on quit/EOF). Shutdown clears core.trace_ctl, releases cur/next, then calls store_fini, which memsets the trace state, so keep that order. 4.6 must emit only through `cur`'s file while it still holds `cur`, and flush after every record, because two append streams may share one file.
 - 4.6: the session hooks and emitter are in repl_trace_emit.c: `repl_trace_session_begin/end(sim)` and `repl_trace_on_decision/on_trace_only(sim, d)`, declared in repl_internal.h. `sim_print_decision` is now exported from sim.h. Session begin stamps `session_no = rs->session_count`, which sim_loop.c increments in `xce_repl_check_begin_round` before `xce_core_begin_session`. 5.3 must call session_begin after a successful begin-session and before signal_go/reconciliation, and session_end after the decision stream ends. Emission moves the record to the store (d->trace=NULL); write errors are reported once per session. policy_name is not stamped; capture falls back to the core picker. repl_trace_emit.c is not yet in sched_sim_isolation_lib or its scan (5.1).
 - 5.1: all 26 tests that include repl.c/sim_loop.c now `#include` the five REPL trace sources (repl_trace_render.c, repl_trace_render_outcome.c, repl_trace_store.c, repl_trace.c, repl_trace_emit.c) and the four core trace sources, exactly once. sched_sim_isolation_lib carries all nine trace sources. test_sim_isolation.c's `test_sim_trace_repl_sources_are_isolated` scans the REPL trace files plus repl_trace_render.h; stdio is allowed there, controller symbols are not. A new REPL trace file must be added to the 26 tests, the lib and the scan. test_sim_startup.c includes only sim.c, so if sim.c starts calling trace code it needs the sources too.
+- 5.2: repl.h now has the interactive-only gates `xce_repl_trace_session_begin/end(sim)` and `xce_repl_check_trace/xce_repl_check_trace_only(sim, d)`; the two check gates also no-op when d->trace is NULL. 5.3 calls these from sim_loop.c. `trace` is routed with the raw line, `info trace` is in usage and help, `set output` checks `repl_trace_file_conflicts` first, and quit/EOF call `repl_trace_shutdown`. The continue-mode rollup prints a traced round on its own via `rs->trace.round_traced`. The baseline transcript changed only in the help and info-usage lines; regenerating also rewrites the .replay fixtures' wall-clock fields, so restore those. Pre-existing: repl_count_jobs (repl.c) never destroys its iterator, leaking 16 B per continue round; the rollup tests run with detect_leaks=0.