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)

Kconfig

@@ -0,0 +1,7 @@
+config ZMK_RGB_EFFECT_LAYER
+    bool "Per-key per-layer RGB effect"
+    depends on ZMK_RGB_UNDERGLOW
+    default n
+    help
+      Enable per-key RGB colors that change based on the active keymap layer.
+      Requires the modular RGB effect API in the ZMK core (rgb-modular branch).