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/behaviors/zmk,behavior-underglow-battery.yaml

@@ -0,0 +1,12 @@
+# Copyright (c) 2024, The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+description: Set underglow color based on battery level
+
+compatible: "zmk,behavior-underglow-battery"
+
+include: two_param.yaml
+
+properties:
+  threshold:
+    type: int

dts/bindings/behaviors/zmk,behavior-underglow-color.yaml

@@ -0,0 +1,8 @@
+# Copyright (c) 2024, The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+description: Set underglow to specified color
+
+compatible: "zmk,behavior-underglow-color"
+
+include: one_param.yaml

dts/bindings/behaviors/zmk,behavior-underglow-indicators.yaml

@@ -0,0 +1,13 @@
+# Copyright (c) 2024, The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+description: Set underglow for num lock indicator
+
+compatible: "zmk,behavior-underglow-indicators"
+
+include: two_param.yaml
+
+properties:
+  indicator:
+    type: int
+    default: 0