Implement shared 7-bag piece sequence

- Add generatePieceBag() and createPieceQueue() for 7-bag system
- Add pieceQueue and playerSequenceIndex to lobby state
- Modify spawnPiece() to consume from shared queue per player
- Update player-hold handler to manage sequence index on hold
- Add sequenceIndex to state updates for tracking
- Update client loadState() to receive sequenceIndex

Co-Authored-By: Claude Opus 4.6 <[email protected]>

public/js/game.js

@@ -366,6 +366,7 @@ class TetrisGame {
     this.level = state.level;
     this.gameOver = state.gameOver;
     this.eliminated = state.eliminated;
+    this.sequenceIndex = state.sequenceIndex || 0;
     this.dropInterval = Math.max(100, 1000 - (this.level - 1) * 100);
   }
 }