feat(viewcube): add compass ring, cardinals, and nav controls

Build the ViewCube out into a full navigation widget:

- Compass ring painted in the cube's ground plane (z = base) so it
  reads as a ground compass and foreshortens with the view.
- N/E/S/W cardinals painted flat onto the ring band; clicking one snaps
  to that side elevation. Added S/W glyphs to the bitmap atlas.
- Home button (top-down view), roll arrows (90° roll about the view
  axis), and four nudge triangles (90° tip/spin) as SVG iced controls
  laid out around the cube. New home.svg.
- WCS / named-UCS selector under the cube, switching the active UCS.

The cube hit area now shares one stack with the control buttons, so a
click on a button is caught by the button while a click on the cube
falls through. Face clicks snap to the highlighted region rather than a
cursor position the viewport's move handler can overwrite.

Camera gains home_view / roll_by / nudge_90; floating viewports drive
the same ops through mutate_active_viewport_camera, which re-encodes the
result to the stored view direction + twist.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

assets/icons/ui/home.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M12 3 L21.5 11.5 H18.5 V20.5 H14 V14.5 H10 V20.5 H5.5 V11.5 H2.5 Z" fill="black"/></svg>