feat(thread): add JobSystem with parallel_for, task_group, dependencies and frame arena

- JobSystem abstraction over the existing ThreadPool worker pool; the
  factory in JobSystem.cpp is the single backend switch point (TBB later).
- Job carries a dependency count and completion callback so a render graph
  can be wired on top without rewriting the scheduler.
- parallel_for(first, last, body, chunk) with static partitioning, auto
  chunk sizing, and a join handle; task_group provides fork/join with
  help-execution so workers can wait without deadlocking.
- Per-frame arena (beginFrame/endFrame) for frame-scoped jobs: no per-frame
  new/delete; frame jobs are recycled by the arena.
- Thread module exposes a shared engine JobSystem (getJobSystem).
- VoxelWorld::remeshDirty now uses parallel_for instead of raw std::thread.
- 24 new zeroerr tests in test/thread.cpp; verified with a standalone MSVC
  harness (24 cases x 30 runs, no flakes).

src/modules/thread/JobSystem.cpp

@@ -0,0 +1,19 @@
+#include "thread/JobSystem.h"
+
+#include "thread/JobSystemThreadPool.h"
+
+namespace eve {
+namespace thread {
+
+/**
+ * Backend switch point: the engine's scheduler is selected here. The default
+ * implementation schedules on top of the existing ThreadPool worker pool. A
+ * TBB backend only needs to implement the JobSystem/Job/TaskGroup interfaces
+ * and be returned from this factory.
+ */
+JobSystem *createJobSystem(int workerCount) {
+    return new JobSystemThreadPool(workerCount);
+}
+
+}  // namespace thread
+}  // namespace eve