feat: implement Phase 40.1 - basic cursor with hjkl movement

Add cursor navigation with hjkl keys (vim-style):

New Module Structure:
- src/useCursor/types.ts: Coord and CursorManager types
- src/useCursor/useCursor.ts: Cursor movement logic
- src/useCursor/useCursorRender.ts: Visual cursor highlight
- src/useCursor/index.ts: Public exports

Cursor Implementation:
- Position tracking with useRef (row/col from 0-8)
- Basic movement: moveLeft, moveRight, moveUp, moveDown
- Bounds checking (prevents moving outside 9x9 grid)
- Keyboard listener for h/j/k/l keys
- Only active when gameStatus === 'started'

Cursor Rendering:
- useCursorRender hook tracks cursor position
- Adds 'cursor' CSS class to current cell
- Removes from previous cell on movement
- Blue highlight with border (rgba(59, 130, 246, 0.3))

Integration:
- src/useGame.ts: Integrate useCursor and useCursorRender
- src/types.ts: Export CursorManager and Coord types
- src/useBoard/types.ts: NEW - BoardManager type
- src/game.css: Add cursor highlight styles

Game Flow:
- Press Space → Game starts → Cursor appears at (0,0)
- Press h/j/k/l → Cursor moves (bounded to grid)
- Visual feedback with blue highlight
- Press Q → Return to waiting state

Phase 40.1 complete. Next: Add number prefixes (5j) and other vim motions.

src/useGame.ts

@@ -2,6 +2,8 @@ import { useState, useEffect } from 'react';
 import type { GameManager, GameOptions } from './types';
 import { useBoard } from './useBoard/useBoard';
 import { useGameStatus } from './useGameStatus';
+import { useCursor } from './useCursor';
+import { useCursorRender } from './useCursor/useCursorRender';
 
 export function useGame(options: GameOptions = {}, platformHook?: unknown): GameManager {
   const { difficulty = 'easy' } = options;
@@ -11,6 +13,11 @@ export function useGame(options: GameOptions = {}, platformHook?: unknown): Game
   const gameStatusManager = useGameStatus();
   const { gameStatus } = gameStatusManager;
   
+  const cursor = useCursor(gameStatusManager);
+  const cursorPosition = cursor.position();
+  
+  useCursorRender(containerRef, cursorPosition);
+  
   const [keyLog, setKeyLog] = useState<{ key: string; timestamp: number }[]>([]);
 
   useEffect(() => {
@@ -30,7 +37,7 @@ export function useGame(options: GameOptions = {}, platformHook?: unknown): Game
     containerRef,
     renderBoard,
     ...gameStatusManager,
-    cursor: {},
+    cursor,
     scoreManager: {
       timeValue: 0,
       keystrokes: 0,