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>

tools/ludic-cli/selfhost.ludic

@@ -125,7 +125,7 @@ function cmd_sh_compile(shbin: pointer, in: pointer, outbin: pointer) -> int {
 function game_build_ok(shbin: pointer, game: pointer, outbin: pointer) -> bool {
   let ll = `{outbin}.ll`
   if not shq(`{shbin} {game} > {ll} 2>{tmp_dir()}/gb.err`) { return false }
-  if not shq(`{cc()} -O2 {ll} -o {outbin} 2>{tmp_dir()}/gb.err`) { return false }
+  if not shq(`{cc()} -O2 {ll}{threads_link_flags(ll)} -o {outbin} 2>{tmp_dir()}/gb.err`) { return false }
   shell(`rm -f {ll}`)
   return true
 }