RGB headsets (e.g. G522) expose HEADSET_RGB_SIGNATURE_EFFECTS (0x0622)
— three firmware-played lighting slots: startup, shutdown, passive.
Each carries an on/off enable, a primary and secondary color, and a
speed.
Add a per-slot setting modeled on the keyboard boot-animation settings
(_RgbBootEffectSetting): a HETERO setting with the enable byte as a
Gtk.Switch plus two color pickers and a speed slider. The rw_class
bridges the firmware's split functions — get/setSignatureEffectParams
(colors + speed) and get/setSignatureEffectState (enable).
Slots are discovered by probing getSignatureEffectState per candidate
(0/1/2), so a device exposing only some slots gets only those
settings. getSignatureEffectsInfo (fn 0) is logged once at debug for a
later move to info-based discovery once its byte layout is confirmed.
Like the boot animations these run autonomously on the device
firmware, so they are not gated on host LED control.
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