feat: implement edit mode state machine (i and Escape)

Added mode state machine with visual feedback:

Mode State:
- CursorMode type: 'normal' | 'edit'
- Tracked in useCursor with useState
- Starts in 'normal' mode

Key Bindings:
- Press 'i' in normal mode → Enter edit mode
- Press 'Escape' in edit mode → Return to normal mode
- All vim motions (hjkl, anchors, etc.) only work in normal mode
- Edit mode blocks all movement keys

Visual Feedback:
- Normal mode: Blue outline (rgb(59, 130, 246))
- Edit mode: Green pulsing outline (rgb(34, 197, 94))
- Smooth pulse animation (1.5s ease-in-out infinite)
- cursor-edit CSS class applied in edit mode

Flow:
1. Game starts in normal mode (blue cursor)
2. Press 'i' → Green pulsing cursor (edit mode)
3. Press 'Escape' → Blue cursor (normal mode)

No digit entry yet - just state machine and visual indicators.
Phase 40.3.1 complete.

src/game.css

@@ -255,7 +255,7 @@
   }
 }
 
-/* Cursor highlight */
+/* Cursor highlight - Normal mode */
 .sudoku-cell.cursor {
   background-color: rgba(59, 130, 246, 0.3);
   outline: 3px solid rgb(59, 130, 246);
@@ -264,6 +264,22 @@
   position: relative;
 }
 
+/* Cursor highlight - Edit mode */
+.sudoku-cell.cursor-edit {
+  background-color: rgba(34, 197, 94, 0.3);
+  outline: 3px solid rgb(34, 197, 94);
+  animation: edit-pulse 1.5s ease-in-out infinite;
+}
+
+@keyframes edit-pulse {
+  0%, 100% {
+    outline-color: rgb(34, 197, 94);
+  }
+  50% {
+    outline-color: rgb(74, 222, 128);
+  }
+}
+
 /* Invalid move flash */
 .sudoku-cell.invalid-move {
   background-color: rgba(239, 68, 68, 0.4);

src/useCursor/index.ts

@@ -1,2 +1,2 @@
 export { useCursor } from './useCursor';
-export type { CursorManager, Coord } from './types';
+export type { CursorManager, Coord, CursorMode } from './types';

src/useCursor/types.ts

@@ -1,5 +1,7 @@
 export type Coord = { row: number; col: number };
 
+export type CursorMode = 'normal' | 'edit';
+
 export type Motion = { dr: number; dc: number; steps: number };
 
 export type VimMotionSystem = {
@@ -13,6 +15,7 @@ export type VimMotionSystem = {
 
 export type CursorManager = {
   position: () => Coord;
+  mode: () => CursorMode;
   moveLeft: (count?: number) => void;
   moveRight: (count?: number) => void;
   moveUp: (count?: number) => void;

src/useCursor/useCursor.ts

@@ -1,10 +1,11 @@
 import { useState, useEffect } from 'react';
-import type { Coord, CursorManager } from './types';
+import type { Coord, CursorManager, CursorMode } from './types';
 import type { GameStatusManager } from '../useGameStatus/types';
 import { useVimMotions } from './useVimMotions';
 
 export function useCursor(gameStatusManager: GameStatusManager, containerRef: React.RefObject<HTMLDivElement | null>) {
   const [position, setPosition] = useState<Coord>({ row: 0, col: 0 });
+  const [mode, setMode] = useState<CursorMode>('normal');
   const { gameStatus } = gameStatusManager;
   const vimMotions = useVimMotions();
 
@@ -92,6 +93,19 @@ export function useCursor(gameStatusManager: GameStatusManager, containerRef: Re
 
     function handleKeyDown(e: KeyboardEvent) {
       const key = e.key;
+
+      if (mode === 'edit') {
+        if (key === 'Escape') {
+          setMode('normal');
+        }
+        return;
+      }
+
+      if (key === 'i') {
+        setMode('edit');
+        return;
+      }
+
       const result = vimMotions.processKey(key);
 
       if (!result) return;
@@ -131,10 +145,11 @@ export function useCursor(gameStatusManager: GameStatusManager, containerRef: Re
 
     window.addEventListener('keydown', handleKeyDown);
     return () => window.removeEventListener('keydown', handleKeyDown);
-  }, [gameStatus]);
+  }, [gameStatus, mode]);
 
   const cursor: CursorManager = {
     position: () => position,
+    mode: () => mode,
     moveLeft,
     moveRight,
     moveUp,

src/useCursor/useCursorRender.ts

@@ -1,11 +1,12 @@
 import { useEffect } from 'react';
-import type { Coord } from './types';
+import type { Coord, CursorMode } from './types';
 import type { BoardManager } from '../useBoard/types';
 import type { GameStatus } from '../useGameStatus/types';
 
 export function useCursorRender(
   containerRef: BoardManager['containerRef'], 
   position: Coord,
+  mode: CursorMode,
   gameStatus: GameStatus,
   boardRendered: number
 ) {
@@ -18,14 +19,18 @@ export function useCursorRender(
     const cells = container.querySelectorAll('.sudoku-cell');
     
     cells.forEach(cell => {
-      cell.classList.remove('cursor');
+      cell.classList.remove('cursor', 'cursor-edit');
     });
 
     const index = position.row * 9 + position.col;
     const targetCell = cells[index];
     
     if (targetCell) {
-      targetCell.classList.add('cursor');
+      if (mode === 'edit') {
+        targetCell.classList.add('cursor', 'cursor-edit');
+      } else {
+        targetCell.classList.add('cursor');
+      }
     }
-  }, [position, containerRef, gameStatus, boardRendered]);
+  }, [position, mode, containerRef, gameStatus, boardRendered]);
 }

src/useGame.ts

@@ -18,8 +18,9 @@ export function useGame(options: GameOptions = {}, platformHook?: unknown): Game
   
   const cursor = useCursor(gameStatusManager, containerRef);
   const cursorPosition = cursor.position();
+  const cursorMode = cursor.mode();
   
-  useCursorRender(containerRef, cursorPosition, gameStatus, boardRendered);
+  useCursorRender(containerRef, cursorPosition, cursorMode, gameStatus, boardRendered);
 
   useEffect(() => {
     renderBoard();