feat(scheduler-core): enforce simulation isolation (task 11.3)

Three-layer regression guard (test_sim_isolation.c) proving sim.h/sim.c/
sim_loop.c hold no controller-facing client/cluster-config/capnp-socket
state: source-text scan, struct-body field scan, and nm(1) link-time
undefined-symbol audit of a new isolated sched_sim_isolation_lib target.

.kiro/specs/scheduler-core/tasks.md

@@ -323,11 +323,12 @@
   - _Depends: 11.1_
   - _Verified: 2026-08-06 — new `sim_loop.c` (562 lines) implements `sched_sim_driver_run()` (replacing task 11.1's `-ENOSYS` placeholder): decodes `.replay` messages in file order, jumps the virtual clock to each `ReplayEvent`'s exact timestamp as it's queued, and on each `ReplaySession` boundary runs a real `core_push_events→core_go→drain decisions→core_end_session` cycle before resetting the batch; mid-stream `SnapshotEntry` messages correctly skipped per AC 20. Discovered task 11.1 left no picker registered and no thread backing the core's cbufs, which would deadlock the decision-drain loop unconditionally the moment a real event loop existed — necessarily in-scope, not a convenience shortcut (independently confirmed by review). Fixed via new `plugin_register_core()` (`plugin.c`/`plugin.h`, registers a picker against a bare `xce_sched_core_t*`, no `xce_scheduler_t*` needed, precedent-matched to task 4.6's `test_core_full_cycle.c`) + `builtin_fcfs_new_plugin()` factory extracted from the existing `register_builtin_fcfs()` (`plugins/builtin_fcfs.c`) + a persistent picker-driving thread (`sim_picker_thread_fn`). Review independently verified the extraction is genuinely additive and behavior-preserving on the LIVE normal-mode path (32/32 whole-project tests including controller/libxce, not just the scheduler suite) — the only finding was a non-blocking doc-accuracy nit (`plugin_register_core()`'s doc comment overclaims parity with the legacy `job_elect_fn` adapter step, which it correctly omits since its only caller never uses that path). Since real resource-matching allocation is unreachable without a genuine `xce_scheduler_t*` (a pre-existing gap belonging to the not-yet-designed scheduler-allocation spec), `sim_picker_thread_fn()` conservatively PENDs every candidate rather than fabricating a DISPATCH — judged by review as honestly-scoped and sufficient for this task's narrow ordering/clock-timing Observable (AC 18's pseudocode drains "decisions" generically, not specifically dispatches), but flagged as a real gap once `--sim`'s decisions need to be meaningful (task 11.4/scheduler-allocation's problem, not this task's). RED phase independently re-reproduced by review (reverted to `-ENOSYS`, confirmed real test failure, restored). 28/28 scheduler suite, 32/32 whole-project, clang-format clean. Non-blocking follow-up noted: the PEND-candidate loop itself has no direct test yet (both new tests exercise the "no resolvable picker" branch only) — worth covering when task 11.4 needs that path exercised for real._
 
-- [ ] 11.3 Enforce simulation isolation
+- [x] 11.3 Enforce simulation isolation
   - The simulation driver's state holds no live-controller client context, no cluster configuration, and no capnp socket of any kind — confirmed by the struct definition itself, not runtime checks alone
   - Observable: a build-time or structural check confirms no controller-facing symbol is reachable from the simulation driver's compiled object
   - _Requirements: 5.17_
   - _Depends: 11.1_
+  - _Verified: 2026-08-06 — new `test_sim_isolation.c` (548 lines) enforces AC 17 across `sim.h`/`sim.c`/`sim_loop.c` in three layers, extending `test_core_isolation.c`'s two-layer pattern rather than duplicating it: (1) source-text scan for forbidden `#include`s and forbidden-symbol call sites, reusing `test_core_isolation.c`'s exact `FORBIDDEN_SYMBOLS` list; (2) a structural check `test_core_isolation.c` has no equivalent of — extracts only the `typedef struct {...} xce_sched_sim_driver_t;` body text (via a comment-stripping pass first, since `sim.h`'s own boundary doc comment legitimately names `xcecli_ctx_t`/`xce_cluster_config_t` in prose) and scans that narrower region for forbidden field types; (3) the literal build/link-time proof — `sim.c` and `sim_loop.c` compiled together into a new isolated `sched_sim_isolation_lib` static_library (`meson.build`), inspected via `nm(1)`'s undefined-symbol table to prove what the compiled object actually calls, independent of transitive header declarations. Deliberately excludes the wall-clock ban (`FORBIDDEN_WALLCLOCK`) — out of this task's Req 5 AC 17 scope, and `sim.c`/`sim_loop.c` legitimately call `clock_gettime()` to seed/advance the core's virtual clock from the driver side, already documented at those call sites. 5/5 new sub-tests pass; 29/29 full scheduler suite passes; whole-project build clean. Independently reviewed and approved.
 
 - [ ] 11.4 Implement decision verification
   - Compare each core-emitted decision against the corresponding recorded decision at the same logical position when verification is enabled; report any mismatch with the logical time and decision detail