Implement Phase 4: pinch-to-zoom and finger pan

- ScaleGestureDetector for pinch zoom (0.5x-4x) with focal-point anchoring
- Finger drag for pan with multi-pointer tracking
- Zoom/pan state managed locally in PadCanvasView for responsiveness,
  synced to EditorViewModel on gesture end

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

PROGRESS.md

@@ -56,9 +56,18 @@ See PROJECT_PLAN.md for the full step list.
 - Used KTX Canvas extensions (withMatrix, withScale, createBitmap) per lint
 - ClickableViewAccessibility suppressed on PadCanvasView (drawing view)
 
+### Phase 4: Zoom and Pan (2026-03-24)
+
+- [x] 4.1: ScaleGestureDetector with focal-point zoom (0.5×–4×)
+- [x] 4.2: Finger drag for pan with multi-pointer tracking (handles finger
+  swaps during pinch gestures)
+- [x] 4.3: Input routing by tool type already in place from Phase 3
+- Zoom/pan state managed locally in PadCanvasView for responsiveness,
+  synced to ViewModel on gesture end
+
 ## In Progress
 
-Phase 4: Zoom and Pan
+Phase 5: Eraser
 
 ## Decisions & Deviations