docs: document Issue 2 fix (cursor border conflicts)

IMPLEMENTATION.md

@@ -476,9 +476,20 @@ const gameManager = {
 - useEffect runs after render
 - boardRendered ensures proper ordering
 
+#### 5. Fixed Cursor Border Conflicts (Issue 2)
+**Problem**: Cursor border conflicting with 3x3 box separators
+- Cells have borders for grid structure
+- Cursor border overlapped/clipped at edges
+- Screenshots showed visual conflicts
+
+**Solution**: Use outline instead of border
+- `outline: 3px solid` (sits above borders)
+- `outline-offset: -3px` (draws inside cell)
+- `z-index: 10 + position: relative` (layers above)
+- Outline doesn't affect layout or conflict with borders
+
 ### Next Steps
-- Issue 2: Fix cursor border conflicts with grid edges
-- Issue 3: Add red flash on invalid movement attempts
+- Issue 3: Add red flash on invalid movement attempts (500ms)
 
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