Undo: a new model is untitled, and undoing it brings the old file back (#214)

geometry.open with "model": "new" cleared the model but kept the session's
file path, so File > Save wrote the new model over the previous one. The path
and the revision saved there (which document the session is) were not part of
the undo state. A step now records the document when its command changed it:
the new model is untitled and unsaved; undo restores the old file and its
clean state, redo the untitled model (or the file it was saved to since).

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

crates/engine/src/cmd/geometry.rs

@@ -14,7 +14,7 @@ use crate::{CommandKind, CommandSpec, EngineError, Result, Session};
 pub const OPEN: CommandSpec = CommandSpec {
     id: "geometry.open",
     label: "Open Design",
-    params: r#"{"path": "part.solidcraft", "model"?: "keep"} — rebuilds the SolidCraft design and makes its bodies the geometry. `model`: "keep" (default) keeps the mesh and every definition, so opening the same design again after an edit picks up the change and sets follow their persistent face names; "new" starts a new model with the design (no mesh, sets, materials, sections, BCs, loads or steps; the unit system stays). The reply's `related` is false when the model (before the open) refers to bodies (mesh tags, set geometry) and the design has none of them, and `stale` lists what no longer resolves: `mesh_bodies` (bodies the mesh is tagged with that the design lacks) and `sets` (sets naming a body or face it lacks). One undo step"#,
+    params: r#"{"path": "part.solidcraft", "model"?: "keep"} — rebuilds the SolidCraft design and makes its bodies the geometry. `model`: "keep" (default) keeps the mesh and every definition, so opening the same design again after an edit picks up the change and sets follow their persistent face names; "new" starts a new model with the design (no mesh, sets, materials, sections, BCs, loads or steps; the unit system stays; the model is untitled until saved, and undo restores the previous file). The reply's `related` is false when the model (before the open) refers to bodies (mesh tags, set geometry) and the design has none of them, and `stale` lists what no longer resolves: `mesh_bodies` (bodies the mesh is tagged with that the design lacks) and `sets` (sets naming a body or face it lacks). One undo step"#,
     form: Some(super::forms::OPEN_DESIGN),
     kind: CommandKind::Model,
     run: open,
@@ -68,6 +68,9 @@ fn open(s: &mut Session, p: &Value) -> Result<Value> {
         s.surfaces.clear();
         s.sources.meshes.clear();
         s.results_view = None;
+        // A new model is untitled: saving asks for a file instead of overwriting the old model's
+        // (undo brings the old file back with the old model).
+        s.set_document(Default::default());
         fit.mesh_bodies.clear();
         fit.sets.clear();
     }