Add performance improvements documentation and game-logic enhancements

- Add IMPROVEMENTS.md with detailed analysis of performance issues and bugs
- Update CLAUDE.md with negative Y overflow explanation
- Update README.md with socket events documentation
- Enhance game-logic.js with improved comments and validation
- Improve server/index.js with better documentation and edge case handling

CLAUDE.md

@@ -59,6 +59,17 @@ cd server && npm run test:coverage
 - `LOBBY_ROOM = 'global-lobby'` - Socket.io room for all players
 - Garbage rules: 2 lines cleared -> 1 garbage row, 3 -> 2, 4 (Tetris) -> 4 rows
 
+### Piece Spawn & Negative Y Overflow
+
+Pieces spawn at `y = 0` (top of visible board). The coordinate system allows **negative Y values** as intentional overflow space:
+
+- **Negative Y allowed**: `isValidPosition()` only checks `newY >= BOARD_HEIGHT`, not `newY < 0`
+- **Purpose**: When garbage is received, pieces are pushed up (y decreases) into negative territory
+- **Player survival**: Player is NOT eliminated when piece goes negative — only when piece LOCKS with blocks in rows 0-1
+- **Recovery**: Piece drops naturally back into visible area; player can continue playing
+
+This design gives players a buffer zone to recover from garbage attacks before elimination.
+
 ### Socket Events
 
 | Event | Direction | Payload |