feat(jobs): real OS threads - Job.parallel_for, fn name, thread-safe Sync

- `fn name` names a top-level function as a value (E_FNREF, lowers to @fn_<name>); the worker
  entry point for Job.parallel_for, which checks it takes (int, pointer-like) and returns void.
- runtime/native/threads.ll (pthreads) and threads_win.ll (Win32 SRWLOCK/CONDITION_VARIABLE): a
  pool of one worker per core but one, parked between batches; every thread claims chunks by
  compare-and-swap. Linked only into programs that use Job/Promise/Sync, by `ludicc -o`,
  `ludic build` and the test suite's build helper.
- Sync.* is real: native mutexes, atomics as cmpxchg retry loops (neither clang takes atomicrw,
  the PC's rejects seq_consistent), mutex-guarded channels, Sync.cpu_count from the OS.
- spawn/despawn on a pool thread stop the program with a located panic.
- examples/library/threads.ludic and its test; docs for fn, Job.parallel_for, Job.is_worker.
- Reseeded (bootstrap-cfree: out.ll == seed.ll). 141/141 on macOS; jobs, threads and the guard
  pass on Windows from the reseeded Windows seed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/language/job/job-is_worker.md

@@ -0,0 +1,19 @@
+---
+id: job-is_worker
+name: Job.is_worker
+category: job
+kind: namespace-method
+tokens: Job.is_worker
+sig: Job.is_worker() -> bool
+tip: True on a Job.parallel_for pool thread, false on the main thread.
+order: 17
+ns: Job
+member: is_worker
+---
+
+True when the code is running on a `Job.parallel_for` pool thread, false on the main thread (and on
+the calling thread while it takes its own share of a batch).
+
+```ludic
+if not Job.is_worker() { print("main thread") }
+```