docs: add webcam compatibility matrix

[JasonColapietro]

Summary

• add a repeatable Windows webcam compatibility matrix covering detection, 720p/30 FPS fallback, mirroring, hand tracking, reconnect, and shutdown
• use the application's exact localized UI and log labels
• distinguish supported close-to-tray behavior from a true exit that releases the webcam
• link the matrix from the contributor guide and exclude captures, serial numbers, device IDs, full logs, and personal imagery

Testing

• markdownlint CONTRIBUTING.md docs/WEBCAM_COMPATIBILITY_MATRIX.md
• relative-link and exact-label assertions
• credential/secret-pattern scan
• git diff --check

Windows/webcam hardware testing was not available locally. The Python suite discovered 28 tests; 17 passed and 11 failed at import because the required project dependencies were absent from the macOS Python 3.14 environment.

Closes #9.