feat(viewport): Ctrl+click opens an entity PE_URL hyperlink

Entities that carry the standard PE_URL XDATA record (what HYPERLINK writes)
can now be followed from the canvas: Ctrl+click opens the link in the system
browser and consumes the click, so no box/lasso selection starts.

- scene::selection::pe_url_of() reads the record with the same rule as the
  Properties panel hyperlink field; unit-tested.
- The ViewCube/compass hit region wins over a hyperlink drawn beneath it.
- Unchanged behaviour when Ctrl is not held or the entity has no link.

src/scene/mod.rs

@@ -46,6 +46,7 @@ mod preview;
 mod project;
 mod scene_markers;
 mod selection;
+pub use selection::pe_url_of;
 pub mod sketch_constraints;
 mod sketch_persist;
 mod sketch_solve;