Add MX-T40 BLE desktop controller and protocol documentation

docs/FEATURES.md

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+# Implemented and potential features
+
+“Verified” below means the owner exercised the control on a physical MX-T40,
+with packet reports or direct observation. It does not establish compatibility
+with other Samsung models.
+
+| Feature | Status |
+| --- | --- |
+| BLE connection/status queries | Verified |
+| EQ presets | Verified, including Rock/Hip-hop/Standard transitions |
+| Bass boost, volume, mute | Verified |
+| Lighting presets and RGB color | Verified; RGB activates COLOR mode |
+| Play/pause and source selection | Owner reports working; playback echo is an event, not absolute state |
+| DJ effects: off, delay, filter, flanger, chorus, wah-wah | Found in Android app; not implemented or tested here |
+| Tempo controls | Found in Android app; not implemented or tested here |
+| USB playback, repeat/shuffle, browsing and track/time information | Found in Android app; not implemented or tested here |
+| Speaker grouping / wireless party controls | Found in Android app; not implemented or tested here |
+| Bluetooth multi-connect setting/status | Found in Android app; model compatibility unverified |
+| Arbitrary per-band EQ | No Samsung app command established |
+
+## Next work
+
+Keep the working transport stable. Add read-only status decoding before new
+controls, then validate one feature at a time on hardware. DJ effects are a
+useful next feature because the app contains both effect selection and level
+commands. Grouping is more involved because it changes relationships between
+speakers and needs more than one device for testing.
+
+The firmware XML is a Qualcomm configuration schema, not a dump of configured
+values or a Samsung command dictionary. It documents audio routing/gain curves,
+button/LED patterns, codec-related definitions, power policies, and GAIA fields.
+Listed schema options do not prove those features are enabled on the MX-T40.