During interpreter exit, threading._shutdown shuts every thread pool
before the package teardown runs while our daemon threads keep working.
map_parallel now runs the items a refused pool did not start on the
calling thread, schedule() drops the path and returns False when its
worker cannot start, and the preview queue skips the file. Each
refusal is logged once at DEBUG without a traceback; other start
failures behave as before.
Vito Sansevero
07474afd46