feat: implement scoring system and win condition (Phase 50)

- Add useTimer hook for elapsed time tracking
- Add useScore hook with comprehensive metrics:
  - Total keystrokes (all keys)
  - Edit keystrokes (only editing keys)
  - Optimal keystrokes (empty cells × 3)
  - Efficiency percentage (optimal/edit × 100)
  - Final score (70% efficiency + 30% time-based)
- Win condition detects board completion only in normal mode
- Track all keystrokes with timestamps in useGame
- Update example app with full scoreboard display
- Expose boardRef in BoardManager for state checking
- Adjust easy mode to 5 removed cells for testing

src/useGame.ts

@@ -4,6 +4,7 @@ import { useBoard } from './useBoard/useBoard';
 import { useGameStatus } from './useGameStatus';
 import { useCursor } from './useCursor';
 import { useCursorRender } from './useCursor/useCursorRender';
+import { useScore } from './useScore';
 
 export function useGame(options: GameOptions = {}, platformHook?: unknown): GameManager {
   const { difficulty = 'easy' } = options;
@@ -19,8 +20,27 @@ export function useGame(options: GameOptions = {}, platformHook?: unknown): Game
   const cursorPosition = cursor.position();
   const cursorMode = cursor.mode();
   
+  const scoreManager = useScore({ board, gameStatusManager, keyLog, cursorMode });
+  
   useCursorRender(containerRef, cursorPosition, cursorMode, gameStatus, boardRendered);
 
+  useEffect(() => {
+    if (gameStatus !== 'started') return;
+
+    function handleKeyDown(e: KeyboardEvent) {
+      setKeyLog(prev => [...prev, { key: e.key, timestamp: Date.now() }]);
+    }
+
+    window.addEventListener('keydown', handleKeyDown);
+    return () => window.removeEventListener('keydown', handleKeyDown);
+  }, [gameStatus]);
+
+  useEffect(() => {
+    if (gameStatus === 'started') {
+      setKeyLog([]);
+    }
+  }, [gameStatus]);
+
   useEffect(() => {
     renderBoard();
   }, []);
@@ -39,11 +59,7 @@ export function useGame(options: GameOptions = {}, platformHook?: unknown): Game
     renderBoard,
     ...gameStatusManager,
     cursor,
-    scoreManager: {
-      timeValue: 0,
-      keystrokes: 0,
-      finalScore: null,
-    },
+    scoreManager,
     keyLog,
     clearKeyLog: () => setKeyLog([]),
     getKeyLog: () => [...keyLog],