fix(exit): stop running jobs before Python joins the executor threads (#376)

ComfyUI returns from main.py after Ctrl+C without closing its event loop,
and Python joins every executor thread before any atexit callback runs, so
a scan batch, thumbnail batch or model download in flight kept ComfyUI
alive until it ended (on Windows each ffmpeg up to its 30 s timeout).

The package now registers a threading._register_atexit hook (after
concurrent.futures', so it runs before that join) that sets an exit signal
and kills the running encoder processes. Job.cancelled follows the signal,
so every check a job already makes between files stops it at exit;
map_parallel starts no further item, the scan walk stops between folders and
a tagging model download stops before its next chunk. Without the internal
hook exit behaves as before.

utils/parallel.py

@@ -9,14 +9,18 @@ to linearly for both jobs.
 
 from concurrent.futures import ThreadPoolExecutor
 
+from .exit_signal import checked
+
 
 def map_parallel(func, items, workers):
     """Apply ``func`` to every item on up to ``workers`` threads, keeping order.
 
     ``workers`` of 1 (or fewer) runs on the calling thread with no pool. An
     exception in ``func`` propagates to the caller, so callers that must
-    survive bad input wrap ``func`` themselves.
+    survive bad input wrap ``func`` themselves. An item that has not started
+    when the interpreter starts exiting raises ExitRequested instead (#376).
     """
+    func = checked(func)
     items = list(items)
     workers = max(1, int(workers or 1))
     if workers == 1 or len(items) <= 1: