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)

include/zmk/events/underglow_color_changed.h

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2024 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <zmk/event_manager.h>
+
+struct zmk_underglow_color_changed {
+    uint32_t layers;
+    bool wakeup;
+};
+
+ZMK_EVENT_DECLARE(zmk_underglow_color_changed);