docs: document Issue 3 fix and Phase 40.1 completion

Documented:
- Issue 3: Red flash on invalid movement (500ms animation)
- Cleanup: Removed all debug console.logs
- Summary: All 3 issues resolved, production-ready

Phase 40.1 complete:
✅ Basic cursor with hjkl movement
✅ Visual cursor highlight (blue outline)
✅ Shows immediately on game start
✅ No border conflicts with grid edges
✅ Red flash feedback on invalid moves
✅ Clean, production-ready code

Ready for next phase or sidequest.

IMPLEMENTATION.md

@@ -488,8 +488,37 @@ const gameManager = {
 - `z-index: 10 + position: relative` (layers above)
 - Outline doesn't affect layout or conflict with borders
 
+#### 6. Added Red Flash on Invalid Movement (Issue 3)
+**Feature**: Visual feedback when attempting invalid moves
+
+**Implementation**:
+- CSS: `.invalid-move` class with red outline and background
+- Animation: 500ms ease-out fade from bright to transparent
+- Logic: `flashInvalidMove()` triggers on bounds violations
+- Automatically removes class after 500ms
+
+**User Experience**:
+- Press h at left edge → Red flash
+- Press j at bottom → Red flash
+- Press k at top → Red flash
+- Press l at right → Red flash
+
+**Cleanup**:
+- Removed all console.log debugging statements
+- Production-ready code
+
+### Summary
+Phase 40.1 complete with all issues resolved:
+- ✅ Basic cursor with hjkl movement
+- ✅ Visual cursor highlight (blue outline)
+- ✅ Shows immediately on game start
+- ✅ No border conflicts with grid
+- ✅ Red flash on invalid movement
+- ✅ Clean, production-ready code
+
 ### Next Steps
-- Issue 3: Add red flash on invalid movement attempts (500ms)
+- Continue Phase 40: Add vim motions (counts, anchors, repeat)
+- Or sidequest before next phase
 
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