画布图层的媒体渲染分派抽成独立函数

- src/components/image-editor/ImageCanvasWorldView.tsx：音频 / 视频 / 序列帧 / model3d / 默认图片这条五层三元链提成 renderCanvasLayerMedia（早返回写成一列判断），渲染分支与传参不变，新增媒体类型不再加深三元

验证：npx vitest run src/components/image-editor/ImageCanvasWorldView.test.tsx（57 用例全绿）、npm run typecheck

src/components/image-editor/ImageCanvasWorldView.tsx

@@ -1024,6 +1024,52 @@ function ImageCanvasImageSequenceLayer({
   );
 }
 
+/**
+ * 图层正文按媒体类型 / 资源类别分派：早返回写成一列判断，新增类型不会再加深三元链。
+ */
+function renderCanvasLayerMedia({
+  layer,
+  mediaTransform,
+  isHovered,
+  onSelectLayer,
+}: {
+  layer: CanvasLayer;
+  mediaTransform?: string;
+  isHovered: boolean;
+  onSelectLayer: () => void;
+}) {
+  if (layer.mediaType === 'audio') {
+    return (
+      <ImageCanvasAudioLayerCard
+        layer={layer}
+        isHovered={isHovered}
+        onSelectLayer={onSelectLayer}
+      />
+    );
+  }
+  if (layer.mediaType === 'video') {
+    return (
+      <ImageCanvasVideoLayer layer={layer} mediaTransform={mediaTransform} />
+    );
+  }
+  if (layer.mediaType === 'image-sequence') {
+    return (
+      <ImageCanvasImageSequenceLayer
+        layer={layer}
+        mediaTransform={mediaTransform}
+      />
+    );
+  }
+  if (layer.assetKind === 'model3d') {
+    return (
+      <ImageCanvasModel3dLayer layer={layer} mediaTransform={mediaTransform} />
+    );
+  }
+  return (
+    <ImageCanvasImageLayer layer={layer} mediaTransform={mediaTransform} />
+  );
+}
+
 const MemoizedCanvasLayerNode = memo(function CanvasLayerNode({
   layer,
   isSelected,
@@ -1078,33 +1124,12 @@ const MemoizedCanvasLayerNode = memo(function CanvasLayerNode({
         onMouseLeave={() => handlers.onLayerMouseLeave(layer.id)}
         aria-label={`选择${layer.title}`}
       >
-        {layer.mediaType === 'audio' ? (
-          <ImageCanvasAudioLayerCard
-            layer={layer}
-            isHovered={isHovered}
-            onSelectLayer={() => handlers.onSelectLayer(layer.id)}
-          />
-        ) : layer.mediaType === 'video' ? (
-          <ImageCanvasVideoLayer
-            layer={layer}
-            mediaTransform={mediaTransform}
-          />
-        ) : layer.mediaType === 'image-sequence' ? (
-          <ImageCanvasImageSequenceLayer
-            layer={layer}
-            mediaTransform={mediaTransform}
-          />
-        ) : layer.assetKind === 'model3d' ? (
-          <ImageCanvasModel3dLayer
-            layer={layer}
-            mediaTransform={mediaTransform}
-          />
-        ) : (
-          <ImageCanvasImageLayer
-            layer={layer}
-            mediaTransform={mediaTransform}
-          />
-        )}
+        {renderCanvasLayerMedia({
+          layer,
+          mediaTransform,
+          isHovered,
+          onSelectLayer: () => handlers.onSelectLayer(layer.id),
+        })}
         <CanvasCardCornerActions
           kindLabel={kindLabel}
           kindAriaLabel={`素材标签：${kindLabel}`}