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/structure/kw-fn.md

@@ -0,0 +1,24 @@
+---
+id: kw-fn
+name: fn
+category: structure
+kind: keyword
+tokens: fn
+sig: fn name
+tip: A top-level function named as a value - the entry point handed to Job.parallel_for.
+order: 9
+---
+
+<code>fn name</code> names a top-level function as a value, so it can be handed to something that calls it later - today, <code>Job.parallel_for</code>, which runs it on worker threads. It is a plain function reference: no closure and nothing captured, so everything the function needs is passed to it (for <code>Job.parallel_for</code>, through its <code>ctx</code> argument). A worker function takes <code>(i: int, ctx)</code> and returns <code>void</code>; the compiler rejects any other shape.
+
+```ludic
+program Squares {
+  function square(i: int, out: words) -> void { out[i] = i * i }
+
+  entry {
+    let sq = words(1000)
+    Job.parallel_for(1000, fn square, sq)
+    print(sq[12])
+  }
+}
+```