refactor: modular RGB effect registration API

Replace hardcoded effect enum/switch-case with STRUCT_SECTION_ITERABLE
registration. Effects register via ZMK_RGB_EFFECT_DEFINE macro.

Core API (rgb_effect.h):
- zmk_rgb_effect_ctx passed to render callbacks
- zmk_rgb_request_refresh() for static effects
- ZMK_RGB_EFFECT_STATIC flag skips animation timer
- on_select/on_deselect lifecycle hooks
- zmk_rgb_hsb_to_rgb/scale helpers now public

Built-in effects (solid/breathe/spectrum/swirl) use same API.
Per-key layer RGB, behaviors, DTS bindings extracted to external
zmk-rgb-effects module.

app/dts/bindings/behaviors/zmk,behavior-underglow-battery.yaml

@@ -1,12 +0,0 @@
-# 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

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

@@ -1,8 +0,0 @@
-# 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

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

@@ -1,13 +0,0 @@
-# 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

app/dts/bindings/zmk,underglow-layer.yaml

@@ -1,24 +0,0 @@
-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