feat: implement digit entry in edit mode (Phase 40.3.3)

- Add setCell method to useBoard to update board state
- Handle digit keys 1-9 in edit mode in useCursor
- Move boardRendered counter to useBoard for proper cursor re-rendering
- Fix golden glow disappearing after digit entry
- Digits display with .user-entered class via renderer

IMPLEMENTATION.md

@@ -762,6 +762,129 @@ Red flash → Stay in Normal
 
 ---
 
-## Iteration 9: [Pending]
+## Iteration 9: Phase 40.3.3 - Digit Entry
+
+**Date**: 2025-01-22
+
+### Goals
+- Allow user to type digits 1-9 when in edit mode
+- Update board state with entered digits
+- Cells show digits with `.user-entered` class
+- Stay in edit mode after entering digit
+
+### Tasks Completed
+
+#### 1. Added setCell Method to BoardManager
+**Updated useBoard.ts:**
+- Added `setCell(row, col, value)` function
+- Updates board string at calculated index
+- Triggers re-render to display change
+
+**Implementation:**
+```typescript
+function setCell(row: number, col: number, value: string): void {
+  const board = boardRef.current.split('');
+  const index = row * 9 + col;
+  board[index] = value;
+  boardRef.current = board.join('');
+  renderBoard();
+}
+```
+
+#### 2. Updated BoardManager Type
+- Added `setCell: (row: number, col: number, value: string) => void;` to type definition
+
+#### 3. Enhanced useCursor for Digit Entry
+**Updated useCursor.ts:**
+- Added `boardManager` parameter (BoardManager)
+- In edit mode, detect digit keys (1-9)
+- Call `boardManager.setCell()` to update board
+- Stay in edit mode (no auto-exit)
+
+**Key Handling:**
+```typescript
+if (mode === 'edit') {
+  if (key === 'Escape') {
+    setMode('normal');
+    return;
+  }
+
+  if (key >= '1' && key <= '9') {
+    boardManager.setCell(position.row, position.col, key);
+    return;
+  }
+
+  return;
+}
+```
+
+#### 4. Updated useGame Integration
+- Pass `board` (BoardManager) to `useCursor()`
+- Three-parameter call: `useCursor(gameStatusManager, containerRef, board)`
+
+### Technical Details
+
+**Board State Flow:**
+```
+User presses '5' in edit mode
+  ↓
+useCursor detects digit key
+  ↓
+Calls boardManager.setCell(row, col, '5')
+  ↓
+Updates boardRef.current string
+  ↓
+Calls renderBoard()
+  ↓
+SudokuRenderer.render() displays digit with .user-entered class
+  ↓
+User sees '5' in the cell
+```
+
+**Cell Classification:**
+- Given cells: Have `.given` class (from initial puzzle)
+- User-entered cells: Have `.user-entered` class (from renderer logic)
+- Empty cells: No value displayed
+
+**Mode Behavior:**
+- Edit mode entered with `i` on empty/user-entered cells
+- Typing 1-9 places digit, stays in edit mode
+- `Escape` exits to normal mode
+- Can navigate to other cells and repeat
+
+### Build Status
+- ✅ TypeScript compiles cleanly
+- ✅ No type errors
+- ✅ Integration complete
+
+### Issue 1: Golden Glow Disappearing After Digit Entry
+
+**Problem**: When typing a digit in edit mode, the `.editing` class (golden glow) disappeared even though we remained in edit mode.
+
+**Root Cause**: `setCell()` calls `renderBoard()` which rebuilds the entire DOM, wiping out the cursor highlight classes.
+
+**Solution**: Moved `boardRendered` counter from `useGame` to `useBoard`
+- Added `boardRendered` state to `useBoard` hook
+- Increment counter at end of `renderBoard()` function
+- Export `boardRendered` from `useBoard`
+- `useCursorRender` depends on `boardRendered` and re-applies cursor classes after DOM rebuild
+
+**Changes**:
+- `useBoard.ts`: Added `useState` for `boardRendered`, increment in `renderBoard()`
+- `useBoard/types.ts`: Added `boardRendered: number` to `BoardManager`
+- `useGame.ts`: Removed local `boardRendered` state, use board's `boardRendered` instead
+- Removed manual `setBoardRendered` calls from `useGame` (automatic now)
+
+**Result**: ✅ Golden glow persists when typing digits in edit mode
+
+### Next Steps
+- Phase 40.3.4: Add erase functionality (x, Backspace, Delete)
+- Phase 40.3.5: Add replace mode (r)
+- Validation and conflict detection
+- Win condition checking
+
+---
+
+## Iteration 10: [Pending]
 
 *To be filled as implementation progresses...*