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/css/modal.css

@@ -678,3 +678,61 @@
     min-width: unset;
     line-height: 1;
 }
+
+/* ── Composite layer editor ────────────────────────────────────── */
+
+#composite-layers-list {
+    display: flex;
+    flex-direction: column;
+    gap: 6px;
+    margin-bottom: 8px;
+}
+
+.composite-layer-item {
+    display: flex;
+    flex-direction: column;
+    gap: 4px;
+    padding: 6px 8px;
+    border: 1px solid var(--border-color);
+    border-radius: 4px;
+    background: var(--card-bg);
+}
+
+.composite-layer-row {
+    display: flex;
+    align-items: center;
+    gap: 6px;
+}
+
+.composite-layer-source {
+    flex: 1;
+    min-width: 0;
+}
+
+.composite-layer-blend {
+    width: 100px;
+    flex-shrink: 0;
+}
+
+.composite-layer-opacity-label {
+    font-size: 0.8rem;
+    white-space: nowrap;
+    flex-shrink: 0;
+}
+
+.composite-layer-opacity {
+    flex: 1;
+    min-width: 60px;
+}
+
+.composite-layer-toggle {
+    flex-shrink: 0;
+}
+
+.composite-layer-remove-btn {
+    font-size: 0.75rem;
+    padding: 0;
+    width: 26px;
+    height: 26px;
+    flex: 0 0 26px;
+}