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

crates/engine/src/cmd/model.rs

@@ -56,6 +56,17 @@ fn units(s: &mut Session, p: &Value) -> Result<Value> {
 
 fn check(s: &mut Session, _: &Value) -> Result<Value> {
     let mut diagnostics = s.model.check();
+    if let Some(design) = s.design.as_deref() {
+        for (body, n) in super::replace::stale(&s.model, design) {
+            diagnostics.push(fieldcraft_model::Diagnostic {
+                severity: Severity::Warning,
+                subject: format!("body `{body}`"),
+                message: format!(
+                    "{n} elements mesh a body the open design no longer has (its partitions changed since it was meshed); mesh.clear removes them"
+                ),
+            });
+        }
+    }
     if let Some(why) = s.mesh_outdated() {
         diagnostics.push(fieldcraft_model::Diagnostic { severity: Severity::Warning, subject: "mesh".into(), message: why });
     }