perf: decode-ahead sprite cache; cache bust v56

Sprites.load() now awaits img.decode() before a tier image counts as
ready, so every sprite is fully decoded during the title screen and
first-draw decode jitter disappears on weak machines. Headless smoke
keeps working via the typeof-decode fallback. Asset URLs bump to v=56.

js/sprites.js

@@ -86,12 +86,12 @@ const Sprites = {
       pending++;
       const img = new Image();
       const finish = () => { if (--pending === 0) G.imagesLoaded = true; };
-      img.onload = () => { this.imgs[name] = img; finish(); };
+      img.onload = () => { this.imgs[name] = img; if (typeof img.decode === 'function') img.decode().then(finish, finish); else finish(); };
       img.onerror = () => {
-        if (hd && !img._sdRetry) { img._sdRetry = true; img.src = 'assets/' + name + '.png?v=55'; return; }
+        if (hd && !img._sdRetry) { img._sdRetry = true; img.src = 'assets/' + name + '.png?v=56'; return; }
         finish();
       };
-      img.src = hd ? 'assets/hd/' + name + '.webp?v=55' : 'assets/' + name + '.png?v=55';
+      img.src = hd ? 'assets/hd/' + name + '.webp?v=56' : 'assets/' + name + '.png?v=56';
     }
   },