engine: a mesh made before its seeds changed is out of date; a remesh names a type change

The setup checklist marks the mesh Outdated (a warm ⚠ in the browser, with
what changed and where to remesh) when seeds, partitions or datum planes
changed after the last mesher ran, and model.check warns (#342). Replacing
a mesh asks with the element type change it makes, such as tet10 → tet4 (#343).

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

crates/engine/src/cmd/model.rs

@@ -55,7 +55,10 @@ fn units(s: &mut Session, p: &Value) -> Result<Value> {
 }
 
 fn check(s: &mut Session, _: &Value) -> Result<Value> {
-    let diagnostics = s.model.check();
+    let mut diagnostics = s.model.check();
+    if let Some(why) = s.mesh_outdated() {
+        diagnostics.push(fieldcraft_model::Diagnostic { severity: Severity::Warning, subject: "mesh".into(), message: why });
+    }
     let errors = diagnostics.iter().filter(|d| d.severity == Severity::Error).count();
     let first_error = diagnostics.iter().find(|d| d.severity == Severity::Error).map(ToString::to_string);
     let progress = crate::progress::to_json(&s.progress_given(first_error));