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

README.md

@@ -17,6 +17,14 @@ When you clear rows, garbage rows are sent to random opponents:
 - 3 lines cleared → 2 garbage rows sent
 - 4 lines (Tetris) → 4 garbage rows sent
 
+### Garbage & Elimination
+
+When you receive garbage, your current piece is pushed upward. Pieces can temporarily exist **above the visible board** (negative Y) — you're not eliminated yet! The piece will drop back down naturally.
+
+**You're only eliminated when:**
+- A piece locks (is placed) with blocks in the top 2 rows (rows 0-1)
+- This happens if too much garbage accumulates and you can't clear rows fast enough
+
 The last player standing wins!
 
 ## Setup