Add MX-T40 BLE desktop controller and protocol documentation

CONTRIBUTING.md

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+# Contributing
+
+Use `uv run --group dev` for tests and formatting; the README lists the commands.
+Keep Tk imports in `gui.py` and protocol behavior in the shared session. Add
+comments for observed firmware quirks and distinguish evidence from inference.
+
+For a protocol change, include a small packet fixture and explain the model,
+transport, requested action, and observed result. Distinguish a packet ACK from
+an actual state report. Never add real Bluetooth addresses or phone identifiers
+to tests; use a synthetic address such as `12:34:56:78:9A:BC`.
+
+Do not commit firmware, APKs, decompiled vendor code, extracted configuration
+files, or full personal Bluetooth traces. Document protocol facts in original
+words and include only the small packet examples needed to reproduce a finding.
+
+Compatibility reports should state speaker model/firmware, Windows version,
+Python version, whether audio playback also worked, and the relevant redacted
+activity log. Tests using fake transports are useful but are not hardware
+compatibility evidence.
+
+Before publishing, choose a license and initialize Git in the clean exported
+folder. GitHub/Gitea repository creation and remote configuration are separate
+from local export; the exporter never uploads anything.