Initial: modular per-key RGB layer effect for ZMK

ZMK module providing per-key per-layer RGB effects via the modular
effect API (zmk_rgb_effect). Extracted from darknao's fork patches.

Contents:
- Layer effect: registers via ZMK_RGB_EFFECT_DEFINE, renders per-key
  colors based on active keymap layer
- Behaviors: underglow_color, underglow_battery, underglow_indicators
- DTS bindings: zmk,underglow-layer + behavior bindings
- Event: underglow_color_changed for reactive refresh

Requires: ZMK core with rgb-modular branch (effect registration API)

dts/bindings/zmk,underglow-layer.yaml

@@ -0,0 +1,24 @@
+description: |
+  Allows defining a rgbmap composed of multiple layers
+
+compatible: "zmk,underglow-layer"
+
+properties:
+  pixel-lookup:
+    type: array
+    required: true
+
+child-binding:
+  description: "A layer to be used in a rgbmap"
+
+  properties:
+    bindings:
+      type: phandle-array
+      required: true
+    layer-id:
+      type: int
+      required: true
+    fade-delay:
+      type: int
+      required: false
+      default: -1