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/types.ts

@@ -1,9 +1,11 @@
 import type { RefObject } from 'react';
 import type { GameStatus, GameStatusManager } from './useGameStatus/types';
+import type { CursorManager } from './useCursor/types';
 
 export type Difficulty = 'easy' | 'medium' | 'hard';
 
 export type { GameStatus, GameStatusManager };
+export type { CursorManager, Coord } from './useCursor/types';
 
 export type GameOptions = {
   difficulty?: Difficulty;
@@ -14,7 +16,7 @@ export type KeyLogEntry = { key: string; timestamp: number };
 export type GameManager = {
   containerRef: RefObject<HTMLDivElement | null>;
   renderBoard: () => void;
-  cursor: any;
+  cursor: CursorManager;
   scoreManager: {
     timeValue: number;
     keystrokes: number;