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);

include/zmk/rgb_underglow_layer.h

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+#include <zmk/keymap.h>
+
+#define ZMK_RGB_CHILD_LEN_PLUS_ONE(node) 1 +
+
+#define ZMK_RGBMAP_LAYERS_LEN                                                                      \
+    (DT_FOREACH_CHILD(DT_INST(0, zmk_underglow_layer), ZMK_RGB_CHILD_LEN_PLUS_ONE) 0)
+
+#define ZMK_RGBMAP_EXTRACT_BINDING(idx, drv_inst)                                                  \
+    {                                                                                              \
+        .behavior_dev = DEVICE_DT_NAME(DT_PHANDLE_BY_IDX(drv_inst, bindings, idx)),                \
+        .param1 = COND_CODE_0(DT_PHA_HAS_CELL_AT_IDX(drv_inst, bindings, idx, param1), (0),        \
+                              (DT_PHA_BY_IDX(drv_inst, bindings, idx, param1))),                   \
+        .param2 = COND_CODE_0(DT_PHA_HAS_CELL_AT_IDX(drv_inst, bindings, idx, param2), (0),        \
+                              (DT_PHA_BY_IDX(drv_inst, bindings, idx, param2))),                   \
+    }
+
+const int rgb_pixel_lookup(int idx);
+const int zmk_rgbmap_id(uint8_t layer);
+const int zmk_rgbmap_fade_delay(uint8_t layer);
+
+const struct zmk_behavior_binding *rgb_underglow_get_bindings(uint8_t layer);
+
+uint8_t rgb_underglow_top_layer_with_state(uint32_t state_to_test);
+uint8_t rgb_underglow_top_layer(void);