Implement Phase 7: rectangle selection with move, copy, delete

- Rectangle selection via stylus drag in select mode
- Visual feedback: dashed selection rect, blue highlight on selected strokes
- Operations: delete selection, drag-to-move, copy (toolbar buttons)
- Full undo support: DeleteMultipleStrokesAction, MoveStrokesAction,
  CopyStrokesAction
- Preallocated RectF to avoid draw-time allocations (lint)

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

PROGRESS.md

@@ -80,9 +80,17 @@ See PROJECT_PLAN.md for the full step list.
 - [x] 6.4: 9 unit tests for UndoManager (perform, undo, redo, depth limit, clear, no-ops)
 - Toolbar now has undo/redo buttons with enabled state
 
+### Phase 7: Selection — Move, Copy, Delete (2026-03-24)
+
+- [x] 7.1–7.2: Rectangle selection with dashed rect and blue highlight
+- [x] 7.3: Delete, drag-to-move, copy operations with toolbar buttons
+- [x] 7.4: Full undo integration — DeleteMultipleStrokesAction, MoveStrokesAction,
+  CopyStrokesAction
+- Drag existing selection to move, tap outside to deselect
+
 ## In Progress
 
-Phase 7: Selection
+Phase 8: Multi-Page Navigation
 
 ## Decisions & Deviations