fix(refboard): guard loadServerPoster against repeated culling calls

Add _serverPosterLoading flag so repeated 200ms culling ticks don't
call textures.load() multiple times before the first resolves. Also
release the texture ref if the sprite was destroyed or started playing
during the async load.

frontend/src/canvas/sprites/VideoSprite.ts

@@ -31,6 +31,7 @@ export class VideoSprite extends Container {
   private videoTexture: Texture | null = null;
   private posterTexture: Texture | null = null;
   private _serverPosterLoaded = false;
+  private _serverPosterLoading = false;
   private _sprite: Sprite | null = null;
   private _shadow: Graphics;
   private _overlay: Graphics;
@@ -99,10 +100,14 @@ export class VideoSprite extends Container {
 
   /** Load server poster. Called by culling system when within poster budget. */
   async loadServerPoster(): Promise<void> {
-    if (!this.posterAssetKey || !this.textures || this.destroyed) return;
+    if (this._serverPosterLoading || !this.posterAssetKey || !this.textures || this.destroyed) return;
+    this._serverPosterLoading = true;
     try {
       const tex = await this.textures.load(this.posterAssetKey);
-      if (this.destroyed || this._isPlaying) return;
+      if (this.destroyed || this._isPlaying) {
+        this.textures!.release(this.posterAssetKey!);
+        return;
+      }
 
       this.posterTexture = tex;
       this._hasPoster = true;
@@ -112,6 +117,8 @@ export class VideoSprite extends Container {
       this._removePlaceholder();
     } catch {
       // Server poster failed — will fall back to client capture if needed
+    } finally {
+      this._serverPosterLoading = false;
     }
   }