geometry.open: choose to keep the model or start a new one, and report what is stale

Opening a design over a model kept its mesh and every definition silently,
even when the design was another part (#145, #114). geometry.open now takes
`model`: "keep" (default: same-design reopen keeps the mesh and sets that
follow persistent face names) or "new" (clears mesh, definitions and
imported surfaces, keeps the unit system). The reply says whether the design
is `related` to the model's geometry references and lists `stale` mesh
bodies and sets. One undo step either way.

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

crates/engine/src/cmd/forms.rs

@@ -799,7 +799,10 @@ pub const TAG: &[Field] = &[
 
 // ---- Geometry and files ----
 
-pub const OPEN_DESIGN: &[Field] = &[path("path", "Design", false).req()];
+pub const OPEN_DESIGN: &[Field] = &[
+    path("path", "Design", false).req(),
+    choice("model", "Model", &[("keep", "Keep the current model"), ("new", "Start a new model")]).def("\"keep\""),
+];
 pub const TESSELLATE: &[Field] = &[reference("body", "Body", C::Body).req(), TOLERANCE];
 pub const IMPORT_MESH: &[Field] = &[path("path", "3MF file", false).req()];
 pub const FILE_OPEN: &[Field] = &[path("path", "Model", false).req()];