Windows quit hardening deferred from #1618: kill order, launch-time tree proof, survivor sweep #1655

#1625 shipped the core of #1618 (waiting for ComfyUI to exit on quit and update). Left for a separate PR with its own Windows QA:

• the Windows kill-order changes;
• the launch-time process-tree proof;
• the quit-time survivor sweep;
• keptAt.

Related, found by #1625's Windows QA: a process started by ComfyUI's interpreter after quit begins (e.g. a custom node's subprocess) is not waited for; listed as a known limit in #1625.