- Added holdPiece and canHold state to TetrisGame class
- Implemented hold() method to swap current piece with held piece
- Added player-hold socket event on server
- Added HOLD preview canvas showing held piece (grayed when unavailable)
- Added C key keyboard shortcut and touch button for hold
- Fixed canHold reset on piece spawn for proper swap functionality
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
