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>

selfhost/main.ludic

@@ -267,6 +267,12 @@ entry {
   # reached by name). Works headless too, so it is outside the windowed block —
   # macOS-only for now, which is where the toolchain runs.
   # On Windows the transport is http_win.ll, over WinHTTP.
+  # Job.* / Promise.* / Sync.* link the OS threads behind Job.parallel_for and Sync.*:
+  # threads.ll (pthreads) or threads_win.ll (Win32).
+  if g_uses_jobs {
+    if g_target_win { cmd = `{cmd} {join_path(home, "runtime/native/threads_win.ll")}` }
+    else { cmd = `{cmd} {join_path(home, "runtime/native/threads.ll")}` }
+  }
   if g_uses_http {
     if g_target_win {
       let httpw = join_path(home, "runtime/native/http_win.ll")