- Extract pure game logic functions to game-logic.js for testability
- Add Jest testing framework with 57 tests covering:
* Unit tests for pure functions (createEmptyBoard, checkBoardOverflow, etc.)
* Integration tests for garbage system (addGarbageToPlayer, sendGarbage)
* Socket.io integration tests for multiplayer flow
- Refactor index.js to use extracted functions
- Tests verify garbage elimination bug fix: players only eliminated when board overflows
