feat: unified composition renderer for clipboard/export

Replace the split snapshot/native renderer paths with a single
composition pipeline (compositionRenderer.ts) that:

- Loads actual source images and uses naturalWidth/Height for
  correct full-resolution sampling (fixes top-left-corner-only bug
  caused by data.w/h being capped to 600px display dimensions)
- Routes image-only selections through native Canvas 2D composition
- Falls back to viewport snapshot for mixed/unsupported selections
  with explicit warnings instead of silent degradation
- Resolves group children via itemResolver for proper group export
- Rejects group children from native composition (local coords
  incompatible with world-space drawing)
- Adds canvas size safety limits with auto-downscale
- Guards VideoSprite._drawFrame against null texture source race
  condition during zoom-triggered culling

New files:
- compositionRenderer.ts — unified composition module
- compositionRenderer.test.ts — 16 tests for entry flattening,
  bounds, dimensions, group handling

Modified:
- clipboard.ts — uses composeSelection() instead of direct renderers
- export.ts — uses composeSelection() + getCompositionDimensions()
- Editor.tsx, useShortcutHandler.ts — pass scene for group resolution
- VideoSprite.ts — null guard on texture source in frame loop

frontend/src/hooks/useShortcutHandler.ts

@@ -80,7 +80,8 @@ export function useShortcutHandler(deps: ShortcutHandlerDeps) {
         writeCanvasToClipboard: async (items?: SceneItem[]) => {
           try {
             const app = canvasRef.current?.getApp() ?? null;
-            await writeCanvasToClipboard(app, viewport, items);
+            const scene = canvasRef.current?.getScene() ?? undefined;
+            await writeCanvasToClipboard(app, viewport, items, scene);
           } catch (err: any) {
             showToast('Copy failed: ' + (err.message || 'clipboard not available'));
           }