Add K8850 per-key RGB LED support

[skarard]

Summary

• Adds per-key RGB LED control for the K8850 (0x8850) 16-key + 3-knob keyboard
• Supports 6 LED modes: off, static, reactive, ripple, rainbow-rows, rainbow-cols
• Per-key color control via YAML config (leds section per layer) and CLI (led command)
• Colors can be specified as named colors (red, green, blue, etc.) or hex #RRGGBB
• LED types and parsing are self-contained in k8850.rs — shared files only pass through opaque serde_yaml::Value

Built on top of #174 (K8850 key binding support) — that PR should be merged first.

LED protocol

[0x03, 0xFE, 0xB0, layer, mode, R, G, B, 16× per-key RGB]

• One 64-byte packet per layer (3 layers total)
• Mode byte: 0=off, 1=static, 2=reactive, 3=ripple, 4=rainbow-rows, 5=rainbow-cols
• Bytes 5-7: base color (R, G, B)
• Bytes 8-55: 16 per-key colors (3 bytes each)

YAML config example

layers:
  - buttons: [...]
    knobs: [...]
    leds:
      mode: "static red"
      colors:
        - ["red", "#FF8000", "yellow", "green"]
        - ["cyan", "blue", "purple", "magenta"]
        - ["white", "#804000", "#008040", "#400080"]
        - ["#FF4040", "#40FF40", "#4040FF", "#C0C0C0"]

Known caveats

• LED freeze/lock (firmware bug): The K8850 firmware has a bug where LEDs freeze after a variable amount of time (observed: 2s–2m18s across multiple tests). This is a pre-existing firmware bug unrelated to this tool — it occurs even after a fresh replug with no USB writes, purely from the firmware's own LED rendering. When frozen:
  • Animated modes (reactive, ripple, rainbow) stop animating
  • Changing layers does not update the LED state
  • Static-on and static-off modes are unaffected in practice, but per-layer LED colors will not update on layer switch while locked
  • A device replug is required to recover
  • This cannot be fixed in software without bootloader/firmware access
• CLI led command overwrites all layers: Each led CLI call sends packets for all 3 layers, setting the target layer and zeroing the others. Use YAML config for multi-layer LED setups.
• leds config is optional: Layers without a leds section are left unchanged. Configs without any leds sections skip LED protocol entirely.

Design

LED-specific types (LedMode, Color, parsers) are fully contained in k8850.rs. Shared files (config.rs, mod.rs, main.rs) only pass through an opaque serde_yaml::Value for the leds field, keeping the cross-cutting diff minimal and allowing other keyboard drivers to define their own LED formats in the future.

Hardware tested

All testing performed on XZKJ-16key_3knob (VID 0x514C / PID 0x8850) by @skarard:

• CLI: static (named colors + hex #RRGGBB)
• CLI: reactive, ripple, rainbow-rows, rainbow-cols, off
• YAML: static per-key colors (named + hex)
• YAML: reactive per-key colors
• YAML: ripple per-key colors
• YAML: multi-layer LEDs (layer 0=red, 1=green, 2=blue — confirmed via layer switch)
• Keybindings + LEDs uploaded together in single config
• LED freeze/lock confirmed (variable: 2s–2m18s, pre-existing firmware bug, replug recovers)

🤖 Generated with Claude Code — reviewed and hardware tested by @skarard