`settings.json` uses a fixed temp name, risking cross-process corruption

[offbyonebit]

Severity: high
Type: bug
Location: clipsync/config.py — Settings._persist_locked() (~line 215)

Problem: _persist_locked() always writes to settings.json.tmp before os.replace(). The tray process and UI child processes can mutate settings concurrently, so two writers can clobber the same temporary file and commit a corrupted mix or lose one update.

Suggested fix: Use a process-unique temporary name such as settings.json.<pid>.tmp (or tempfile in the same directory) so concurrent writers cannot collide.

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_{Filed from a full-source audit of the repo (2026-07-29). Independently confirmed against the source before filing.}