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/include/linker/zmk-rgb-effects.ld

@@ -0,0 +1,9 @@
+/*
+ * Copyright (c) 2024 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <zephyr/linker/linker-defs.h>
+
+ITERABLE_SECTION_ROM(zmk_rgb_effect, 4)