feat(tetris-attack): load custom panel artwork sprite sheet

Replace procedural panel rendering with custom painted artwork
(tetrisattack-panels.png) for Tetris Attack game.

data/tetrisattack-artwork.json

@@ -2,7 +2,7 @@
   "_comment": "Drop-in painted art for Tetris Attack (spec: src/games/tetrisattack/sprites.md). Paint a sheet, put it under assets/images/tetrisattack/, set its path below and reload — no code changes needed. Any sheet left path:null renders procedurally (colored panel shapes; opponent-portrait fallback for characters).",
   "panelSheet": {
     "key": "tetrisattack-panels",
-    "path": null,
+    "path": "assets/images/tetrisattack/tetrisattack-panels.png",
     "frameWidth": 48,
     "frameHeight": 48
   },