Implement Phase 5: stroke-level eraser

- Eraser mode: stylus touch/drag hit-tests against stroke bounding boxes
  expanded by 42pt radius (~3.5mm), processes historical points
- Deletes hit strokes from canvas view and Room DB
- Backing bitmap rebuilt automatically on stroke removal

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

PROGRESS.md

@@ -65,9 +65,15 @@ See PROJECT_PLAN.md for the full step list.
 - Zoom/pan state managed locally in PadCanvasView for responsiveness,
   synced to ViewModel on gesture end
 
+### Phase 5: Eraser (2026-03-24)
+
+- [x] 5.1–5.3: Stroke-level eraser with bounding box hit test (42pt radius),
+  processes historical touch points for thorough erasing, deletes from
+  view + Room DB, rebuilds backing bitmap
+
 ## In Progress
 
-Phase 5: Eraser
+Phase 6: Undo/Redo
 
 ## Decisions & Deviations