Add suport to QMK Raw mode

[ddrcode]

When the user changes the layer on their QMK keyboard (via momentary or toggle), Charon could receive this event over Raw HID and display an on-screen overlay with the corresponding keymap. Would help recall esoteric mappings and make layer usage more discoverable. Overlay could fade out after timeout or keypress. Long-term: telemetry-driven heatmaps, editable maps, and many more features.

As part o this ticket:

• explore communication between QMK and Charon
• add auto-discovery if Raw mode is enabled
• add layer change recognition
• display keymap (static one - for this POC)

[ddrcode]

Enabled RAW_MODE on my test keyboard (Keychron Q10) and added code that sets layer number on layer change.

[ddrcode]

Partially implemented as part of #31

Added features:

• connectivity with raw device
• defined and documented communication protocol
• switching/toggling Charon mode via raw mode (no shortcut needed)

[ddrcode]

New features added to in #40

• Layer change discovery
• Dynamic keymap preview (based on JSON file provided)