Add composite color strip source type with layer blending

Composite sources stack multiple existing color strip sources as layers
with configurable blend modes (Normal, Add, Multiply, Screen) and per-layer
opacity. Includes full CRUD, hot-reload, delete protection for referenced
layers, and pre-allocated integer blend math at 30 FPS.

Also eliminates per-frame numpy allocations in color_strip_stream,
effect_stream, and wled_target_processor (buffer pre-allocation).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/js/app.js

@@ -91,6 +91,7 @@ import {
     showCSSEditor, closeCSSEditorModal, forceCSSEditorClose, saveCSSEditor, deleteColorStrip,
     onCSSTypeChange, onEffectTypeChange, onAnimationTypeChange, updateEffectPreview,
     colorCycleAddColor, colorCycleRemoveColor,
+    compositeAddLayer, compositeRemoveLayer,
     applyGradientPreset,
 } from './features/color-strips.js';
 
@@ -281,6 +282,8 @@ Object.assign(window, {
     updateEffectPreview,
     colorCycleAddColor,
     colorCycleRemoveColor,
+    compositeAddLayer,
+    compositeRemoveLayer,
     applyGradientPreset,
 
     // calibration