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>

src/ui/icons.rs

@@ -12,6 +12,8 @@ use iced::{Color, Element, Length, Theme};
 const TRI_DOWN: &[u8] = include_bytes!("../../assets/icons/ui/tri_down.svg");
 const TRI_UP: &[u8] = include_bytes!("../../assets/icons/ui/tri_up.svg");
 const TRI_RIGHT: &[u8] = include_bytes!("../../assets/icons/ui/tri_right.svg");
+const TRI_LEFT: &[u8] = include_bytes!("../../assets/icons/ui/tri_left.svg");
+const HOME: &[u8] = include_bytes!("../../assets/icons/ui/home.svg");
 const UNDO: &[u8] = include_bytes!("../../assets/icons/ui/undo.svg");
 const REDO: &[u8] = include_bytes!("../../assets/icons/ui/redo.svg");
 
@@ -166,6 +168,16 @@ pub fn arrow_right<'a, M: 'a>(size: f32, color: Color) -> Element<'a, M> {
     icon(TRI_RIGHT, size, color)
 }
 
+/// Leftward caret.
+pub fn arrow_left<'a, M: 'a>(size: f32, color: Color) -> Element<'a, M> {
+    icon(TRI_LEFT, size, color)
+}
+
+/// House glyph — the ViewCube "home view" button.
+pub fn home<'a, M: 'a>(size: f32, color: Color) -> Element<'a, M> {
+    icon(HOME, size, color)
+}
+
 /// Caret that flips up/down with `open`.
 pub fn arrow_toggle<'a, M: 'a>(open: bool, size: f32, color: Color) -> Element<'a, M> {
     if open {