Update docs for DC-1 testing, fix toolbar clipping

- Updated DESIGN.md: rendering strategy (no backing bitmap, screen-space
  grid, viewport clamping), added line snap and box tool documentation
- Updated PROGRESS.md: DC-1 testing results and fixes applied
- Updated CLAUDE.md: key conventions for rendering
- Fix toolbar overlap: use clipToBounds on canvas AndroidView to prevent
  it from drawing over the toolbar area

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

DESIGN.md

@@ -181,20 +181,39 @@ The editor toolbar controls the active mode:
 | Draw     | Create strokes with the active pen size               |
 | Erase    | Touch a stroke to delete it (stroke-level eraser)     |
 | Select   | Draw a rectangle to select strokes, then move/copy/delete |
+| Box      | Drag corner-to-corner to draw rectangles              |
+
+### Line Snap
+
+When drawing in pen mode, holding the stylus still for 1.5 seconds
+activates line snap mode. The stroke is replaced with a straight line
+from the original pen-down point to the current position. Moving the
+pen continues the straight line. The snap timer is canceled if the pen
+moves more than ~5mm (60pt at 300 DPI) from the origin before the
+timer fires. This allows drawing straight lines for diagrams without
+a separate ruler tool.
 
 ## Rendering Strategy
 
 ### On-Screen
 
-1. **Completed strokes** are rendered to a backing `Bitmap`. This bitmap is
-   redrawn only when strokes change (add, delete, move).
-2. **In-progress stroke** (pen is down) is drawn directly to the canvas on
-   each `onDraw` call, on top of the backing bitmap.
-3. **Grid** is drawn as a separate pass — thin gray lines at 60pt intervals.
-4. The view `Matrix` transforms everything from canonical to screen space.
+1. **Completed strokes** are drawn directly as `Path` objects on each
+   `onDraw` call. No backing bitmap — direct path rendering avoids the
+   resolution loss that caused blurry strokes on e-ink displays.
+2. **In-progress stroke** (pen is down) is drawn on top of completed strokes.
+3. **Grid** is drawn in screen pixel space (not canonical space) with
+   pixel-snapped line positions. This ensures perfectly uniform square
+   spacing regardless of zoom level. Grid uses 1px screen-space lines.
+4. **Strokes** are drawn in canonical space via the view `Matrix`.
+   Anti-aliasing is disabled on all paint objects for crisp e-ink rendering.
 
-This approach means `onDraw` is fast for the common case (pen moving):
-draw the cached bitmap + one active path + grid.
+### Viewport
+
+The page always fills the entire visible canvas area:
+- Minimum zoom is computed dynamically so the page never appears smaller
+  than the viewport in either dimension.
+- Pan is clamped so page edges stay at or beyond viewport edges.
+- Background is white to eliminate any visible non-page area.
 
 ### PDF Export
 
@@ -220,6 +239,7 @@ Command pattern with a depth limit of 50:
 | `DeleteStrokeAction` | Delete stroke     | Re-insert stroke   |
 | `MoveStrokesAction`  | Offset points     | Offset back        |
 | `DeleteMultipleAction`| Delete strokes   | Re-insert strokes  |
+| `CopyStrokesAction`  | Duplicate strokes | Delete copies      |
 
 The `UndoManager` maintains two stacks. Performing a new action clears the
 redo stack. Each action also persists or deletes from Room as part of
@@ -244,7 +264,6 @@ Stroke-level eraser (not pixel-level):
 2. Hit test: check stroke bounding box first (fast reject), then check
    point-by-point distance (threshold: ~42 canonical points / ~3.5mm).
 3. Hit strokes are deleted immediately (with undo support).
-4. Backing bitmap is rebuilt after deletion.
 
 ## E-ink Considerations