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/Kconfig

@@ -322,8 +322,8 @@ config ZMK_RGB_UNDERGLOW_SPD_START
     range 1 5
 
 config ZMK_RGB_UNDERGLOW_EFF_START
-    int "RGB underglow start effect int value related to the effect enum list"
-    range 0 4
+    int "RGB underglow start effect index (clamped to available effects at runtime)"
+    range 0 15
 
 config ZMK_RGB_UNDERGLOW_ON_START
     bool "RGB underglow starts on by default"
@@ -335,10 +335,6 @@ config ZMK_RGB_UNDERGLOW_AUTO_OFF_USB
     bool "Turn off RGB underglow when USB is disconnected"
     depends on USB_DEVICE_STACK
 
-config EXPERIMENTAL_RGB_LAYER
-    bool "Experimental per-key per-layer RGB underglow"
-    default n
-
 endif # ZMK_RGB_UNDERGLOW
 
 menuconfig ZMK_BACKLIGHT