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/frontend/ast.ludic

@@ -37,6 +37,7 @@ const E_TRY:    int = 52         # try EXPR else { ... } — recover a fallible
                                  #   a=the fallible (result-typed) expression  b=else block (its
                                  #   trailing expression is the fallback)  line=source line
 const E_LIST:   int = 54         # [a, b, c] — a slice literal; kids=the elements, all of one type
+const E_FNREF:  int = 55         # fn name — a top-level function as a value (s=the name); a worker entry point
 # statements
 const S_LET:      int = 10
 const S_ASSIGN:   int = 11