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/sync/sync-cpu_count.md

@@ -5,13 +5,14 @@ category: sync
 kind: namespace-method
 tokens: Sync.cpu_count
 sig: Sync.cpu_count() -> int
-tip: Worker lanes available to the scheduler.
+tip: The machine's logical cores (1 to 64); Job.parallel_for uses one thread per core.
 order: 14
 ns: Sync
 member: cpu_count
 ---
 
-Worker lanes available to the scheduler.
+The machine's logical cores, from 1 to 64. `Job.parallel_for` runs on that many threads: a pool of
+one per core but one, plus the caller.
 
 ```ludic
 let lanes = Sync.cpu_count()