engine: pick-driven points, cells' methods per cell, rigid walls as interactions

- geometry.bodies lists each body's vertices; geometry.datum.point by
  vertex takes the design's vertex names (labelled Vertex, not Mesh
  vertex) (#346).
- geometry.centre: the point for what is picked: a vertex, nodes, a
  hole's centre (cylindrical faces or circular edges), else a centroid
  (#351).
- mesh.cells' form: the cells first, and a method drop-down per cell
  (schema kind RefChoices) instead of JSON (#344).
- interaction.rigid_wall(.delete): rigid walls of an explicit step from
  the Interactions tab, no longer coming soon (#350).

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

crates/engine/src/cmd/mod.rs

@@ -42,6 +42,7 @@ pub static ALL: &[CommandSpec] = &[
     model::INFO,
     geometry::OPEN,
     geometry::BODIES,
+    geometry::CENTRE,
     geometry::TESSELLATE,
     geometry::IMPORT_MESH,
     datum::DATUM_PLANE,
@@ -107,6 +108,8 @@ pub static ALL: &[CommandSpec] = &[
     contact::TIE,
     contact::TIE_EDIT,
     contact::TIE_DELETE,
+    contact::RIGID_WALL,
+    contact::RIGID_WALL_DELETE,
     define::MASS_CREATE,
     define::MASS_EDIT,
     define::MASS_DELETE,