fix: move IN-loop clipboard ops off watchdog thread pool (Windows)

[offbyonebit]

Root cause

On Windows, Observer uses WindowsApiObserver, which in watchdog 4.x dispatches filesystem events through an internal thread pool. The previous code called _on_file_changed() synchronously inside the watchdog event handler, meaning every incoming file-change event blocked a pool worker thread while doing:

1. File read from disk
2. pyperclip.paste() — Windows clipboard access, a GUI resource that serializes between processes and can stall
3. pyperclip.copy() — same

Under normal Syncthing activity (many temp-file events), all pool workers could be blocked simultaneously → BrokenExecutor / pool-exhaustion error + the app becoming sluggish or unresponsive. Linux uses InotifyObserver which doesn't have a thread pool limit in the same way, so the bug didn't surface there.

There was also a secondary performance issue: _matches() called Path.resolve() three times on every filesystem event (including all the Syncthing temp files), which is expensive on Windows.

Changes (clipsync/clipboard.py)

• _in_queue + clipsync-in thread: watchdog handler now just does queue.put(path) and returns immediately, keeping the pool free. A dedicated thread we own drains the queue and calls _on_file_changed().
• Cached resolved paths in _ClipboardFileHandler.__init__: resolved once at startup, reused on every event.
• Fast name pre-filter in _matches(): checks Path(path).name against a set {"clipboard.txt", "clipboard.png"} before the more expensive Path.resolve() call, so Syncthing temp-file events are rejected in O(1).
• Proper _in_loop shutdown: stop() posts a sentinel ("") to unblock the queue get, joins the thread with a 3 s timeout.

Test plan

• On Windows: copy text between two peers — should be fast with no pool errors in the log
• Confirm clipsync-in thread appears in thread listing at startup
• Confirm clean shutdown (no hang during stop())
• Existing test suite passes (ruff, mypy, pytest)

https://claude.ai/code/session_0128MyVVEu1Dt3jTeaZ1edyL

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