fix: harden GIF support — ref-counted sources, defensive detection, play budget

1. Move GIF loading behind TextureManager.loadGif/releaseGif with
   ref-counting (fixes shared-source invalidation on duplicate GIFs)
2. Strip query strings/fragments before .gif extension check
3. GIFs now load paused — separate MAX_PLAYING_GIFS=8 budget in
   culling system, only nearest N animate (others show static frame)

frontend/src/canvas/SceneManager.ts

@@ -27,6 +27,18 @@ import type {
   SceneObject,
 } from './scene-format';
 
+// ---------------------------------------------------------------------------
+// GIF detection
+// ---------------------------------------------------------------------------
+
+/** Check if an asset key points to a GIF file.
+ *  Strips query strings and fragments before checking extension. */
+function isGifAsset(asset: string): boolean {
+  // Strip query string and fragment
+  const clean = asset.split('?')[0].split('#')[0];
+  return clean.toLowerCase().endsWith('.gif');
+}
+
 // ---------------------------------------------------------------------------
 // SceneItem
 // ---------------------------------------------------------------------------
@@ -276,7 +288,7 @@ export class SceneManager {
     switch (data.type) {
       case 'image': {
         const imgData = data as ImageObject;
-        if (imgData.asset.toLowerCase().endsWith('.gif')) {
+        if (isGifAsset(imgData.asset)) {
           displayObject = new AnimatedGifSprite(imgData.asset, imgData.w, imgData.h, this.textures);
         } else {
           displayObject = new ImageSprite(imgData.asset, imgData.w, imgData.h, this.textures);