fix: harden daily Windows behavior

[JigSawPT]

Summary

• prevent duplicate instances with a per-session Windows mutex
• position the Swap overlay correctly on monitors with negative coordinates
• add opt-in privacy-conscious diagnostics
• expand deterministic coverage from 8 to 15 tests

Validation

• 15 pytest tests passed
• compileall and uv lock checks passed
• PyInstaller one-file build completed
• visual hot-corner and overlay test passed at 150% DPI
• real VLC to Calculator foreground transition passed
• second executable launch exited without another resident instance
• frozen startup shortcut logic passed with a path containing spaces