feat: implement erase functionality (Phase 40.3.5)

- Add x, d, Delete in normal mode to erase user-entered cells
- Add Backspace in edit mode to erase and stay in edit mode
- Add Backspace in normal mode as invalid move (red flash)
- All erase commands respect given cell protection
- Empty cells flash red on erase attempt in normal mode

IMPLEMENTATION.md

@@ -982,12 +982,115 @@ Back in normal mode (blue cursor)
 - ✅ All validation rules working correctly
 
 ### Next Steps
-- Phase 40.3.5: Add erase functionality (x, Backspace, Delete)
 - Validation and conflict detection
 - Win condition checking
 
 ---
 
-## Iteration 11: [Pending]
+## Iteration 11: Phase 40.3.5 - Erase Functionality
+
+**Date**: 2025-01-22
+
+### Goals
+- Implement erase commands to clear user-entered digits
+- Different behavior in normal vs edit mode
+- Always respect given cell protection
+
+### Tasks Completed
+
+#### 1. Normal Mode Erase (x, d, Delete)
+**Keys: `x`, `d`, `Delete`**
+- Only work on user-entered cells
+- Set cell value to '.' (empty)
+- Flash red if cell is given or already empty
+
+**Validation Logic:**
+```typescript
+if (key === 'x' || key === 'd' || key === 'Delete') {
+  const isGiven = targetCell.classList.contains('given');
+  const isUserEntered = targetCell.classList.contains('user-entered');
+
+  if (isGiven || !isUserEntered) {
+    flashInvalidMove();
+    return;
+  }
+
+  boardManager.setCell(position.row, position.col, '.');
+  return;
+}
+```
+
+#### 2. Edit Mode Erase (Backspace)
+**Key: `Backspace`**
+- Works in edit mode only
+- Erases current cell value
+- Flash red if cell is given
+- Stay in edit mode after erasing
+
+**Implementation:**
+```typescript
+if (key === 'Backspace') {
+  const targetCell = cells[index];
+
+  if (targetCell && targetCell.classList.contains('given')) {
+    flashInvalidMove();
+    return;
+  }
+
+  boardManager.setCell(position.row, position.col, '.');
+  return;
+}
+```
+
+#### 3. Normal Mode Backspace Protection
+**Key: `Backspace` in normal mode**
+- Always triggers red flash (invalid move)
+- Prevents accidental cell clearing in normal mode
+- User must use x, d, or Delete explicitly
+
+### Erase Command Rules
+
+| Mode | Key | Empty Cell | User-Entered Cell | Given Cell |
+|------|-----|------------|-------------------|------------|
+| Normal | `x`, `d`, `Delete` | ❌ Red flash | ✅ Erase to '.' | ❌ Red flash |
+| Normal | `Backspace` | ❌ Red flash | ❌ Red flash | ❌ Red flash |
+| Edit | `Backspace` | ✅ Erase (noop) | ✅ Erase to '.' | ❌ Red flash |
+
+### Technical Details
+
+**Cell State After Erase:**
+- Board string updated with '.' at cell index
+- Cell loses `.user-entered` class on next render
+- Cell becomes empty and eligible for `i` command again
+
+**Flow Example:**
+```
+User enters '5' in cell with 'i'
+  ↓
+Cell has .user-entered class, displays '5'
+  ↓
+Press Escape → normal mode
+  ↓
+Press 'x' or 'd' or Delete
+  ↓
+boardManager.setCell(row, col, '.')
+  ↓
+Board re-renders, cell is empty
+  ↓
+Cell loses .user-entered, can use 'i' again
+```
+
+### Build Status
+- ✅ TypeScript compiles cleanly
+- ✅ No type errors
+- ✅ All erase commands working correctly
+
+### Next Steps
+- Validation and conflict detection
+- Win condition checking
+
+---
+
+## Iteration 12: [Pending]
 
 *To be filled as implementation progresses...*

src/useCursor/useCursor.ts

@@ -112,6 +112,50 @@ export function useCursor(gameStatusManager: GameStatusManager, containerRef: Re
           return;
         }
 
+        if (key === 'Backspace') {
+          const container = containerRef.current;
+          if (!container) return;
+
+          const cells = container.querySelectorAll('.sudoku-cell');
+          const index = position.row * 9 + position.col;
+          const targetCell = cells[index];
+
+          if (targetCell && targetCell.classList.contains('given')) {
+            flashInvalidMove();
+            return;
+          }
+
+          boardManager.setCell(position.row, position.col, '.');
+          return;
+        }
+
+        return;
+      }
+
+      if (key === 'x' || key === 'd' || key === 'Delete') {
+        const container = containerRef.current;
+        if (!container) return;
+
+        const cells = container.querySelectorAll('.sudoku-cell');
+        const index = position.row * 9 + position.col;
+        const targetCell = cells[index];
+
+        if (!targetCell) return;
+
+        const isGiven = targetCell.classList.contains('given');
+        const isUserEntered = targetCell.classList.contains('user-entered');
+
+        if (isGiven || !isUserEntered) {
+          flashInvalidMove();
+          return;
+        }
+
+        boardManager.setCell(position.row, position.col, '.');
+        return;
+      }
+
+      if (key === 'Backspace') {
+        flashInvalidMove();
         return;
       }