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-parallel_for.md

@@ -0,0 +1,34 @@
+---
+id: job-parallel_for
+name: Job.parallel_for
+category: job
+kind: namespace-method
+tokens: Job.parallel_for
+sig: Job.parallel_for(count, work, ctx) -> void
+tip: Run work(i, ctx) for every i in [0, count) across all cores; returns when every call is done.
+order: 16
+ns: Job
+member: parallel_for
+---
+
+Run `work(i, ctx)` for every `i` in `[0, count)` on real OS threads, and return when every call has
+finished. The indices are split into chunks shared by a worker pool (one thread per core but one,
+started on first use) and the calling thread, so each index runs exactly once, in no particular order.
+
+`work` is a top-level function taking `(i: int, ctx)`, named with `fn`; `ctx` is whatever it computes
+on (`words`, `bytes` or a `pointer`). A worker computes on what it was handed and writes only its own
+index's results: it must not `spawn`, `despawn`, `push` onto a list another thread can see, or use
+Http or Audio. `spawn` and `despawn` on a worker stop the program with a located message. Shared
+counters go through `Sync.add`, shared totals behind a `Sync.mutex`.
+
+```ludic
+program Squares {
+  function square(i: int, out: words) -> void { out[i] = i * i }
+
+  entry {
+    let sq = words(20000)
+    Job.parallel_for(20000, fn square, sq)
+    print(sq[141])
+  }
+}
+```