geometry: meshes go with the bodies a partition change removes (#385)

Deleting, suppressing or editing a partition (or its datum), or cutting a
meshed body, removes the meshes of the bodies that are gone, reported as
replaced, in the same undo step. model.check lists meshes of partition
cells the open design no longer has (a model saved before the cut
changed), and mesh.clear removes them, or the meshes of named bodies.

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

docs/features.md

@@ -47,7 +47,7 @@ the control channel and AI agents over MCP (docs/mcp.md) run the same commands.
 | Improve | Untangle — Repair inverted elements | coming soon (M6) |  |
 | Improve | Layers — Boundary padding and layers through thin walls | coming soon (M6) |  |
 | Edit | Node — Add a node | ready | `mesh.node.add`, `mesh.node.remove` |
-| Edit | Element — Add an element on existing nodes | ready | `mesh.element.add`, `mesh.element.remove` |
+| Edit | Element — Add an element on existing nodes | ready | `mesh.element.add`, `mesh.element.remove`, `mesh.clear` |
 | Edit | Mirror Mesh — Mirror a body's mesh across a plane, joined on the plane (mesh half a symmetric part) | ready | `mesh.mirror` |
 | Edit | Merge Nodes — Merge coincident nodes, keeping sets, loads and boundary conditions on the survivors; check for duplicates and cracks, renumber | ready | `mesh.node.merge`, `mesh.check.duplicates`, `mesh.renumber` |
 | Edit | Remesh Region — Remesh a region in place; split and collapse elements | coming soon (M6) |  |