ui: pick vertices, points and cells from the viewport

- Select: Vertices picks the design's B-rep vertices (meshed or not), and
  a selected vertex is marked (#346).
- Use selection on Datum Point (vertex, edge, face, or a hole's centre)
  and Reference Point (geometry.centre: a hole's centre, a vertex, nodes)
  (#346, #351).
- Partitioned Mesh: Use selection takes the picked bodies as cells; the
  cell list has All, None and Invert; hovering a cell's row (or its method
  row) highlights the cell in the viewport (#344).
- Rigid Wall's form takes its set from the selection; the Interactions
  branch lists the explicit steps' rigid walls, each opening its step
  (#350).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/ui-egui/src/scene.rs

@@ -194,7 +194,7 @@ mod tests {
         let mut d = DisplayMesh::default();
         // Two edges of 10 mm; the first in two segments.
         d.add_curves(&[vec![[0.0; 3], [4.0, 0.0, 0.0], [10.0, 0.0, 0.0]], vec![[0.0; 3], [0.0, 10.0, 0.0]]]);
-        let names = [crate::selection::BodyNames { name: "B".into(), faces: vec![], edges: vec!["a|b".into(), "a|c".into()] }];
+        let names = [crate::selection::BodyNames { name: "B".into(), faces: vec![], edges: vec!["a|b".into(), "a|c".into()], vertices: Vec::new() }];
         let mut m = Model::default();
         let edge = GeometryRef::new("B", Entity::Edge("a|b".into())).unwrap();
         m.set_seed(Seed::new(edge, Divisions::Count(4), Some(Bias { ratio: 8.0, toward: BiasToward::Start }), SeedConstraint::Exact).unwrap())