engine: a prescribed displacement or temperature satisfies the loads stage (#356)

The setup checklist asked for a load on rubber-seal, driven only by a
prescribed displacement, while model.check called it ready.

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

crates/engine/src/progress.rs

@@ -128,6 +128,8 @@ impl Session {
         // Models without elements (point masses, fluid domains) need no material or section.
         let elementless = analysable && m.elements().is_empty();
         let modal_only = !m.steps().is_empty() && m.steps().iter().all(|st| matches!(st.procedure, Procedure::Frequency { .. }));
+        // A model driven by a prescribed displacement or temperature needs no load either.
+        let driven = m.boundary_conditions().values().any(|bc| bc.drives());
         let mut next_given = false;
         Stage::ALL
             .into_iter()
@@ -137,7 +139,7 @@ impl Session {
                 let optional = match stage {
                     Stage::Prepare => true,
                     Stage::Material | Stage::Section => elementless,
-                    Stage::Loads => modal_only,
+                    Stage::Loads => modal_only || driven,
                     _ => false,
                 };
                 let command = r.provider();
@@ -234,6 +236,17 @@ mod tests {
         assert_eq!((status(&s, Stage::Results), to_json(&s.progress())["next"]["command"].clone()), (Next, json!("results.contour")));
     }
 
+    /// A model driven by a prescribed displacement (rubber-seal) needs no load: the checklist
+    /// agrees with the model check and offers the solve next.
+    #[test]
+    fn a_displacement_driven_model_needs_no_load() {
+        let mut s = Session::default();
+        s.run_script(&crate::sample::rubber_seal()).unwrap();
+        assert!(s.model.loads().is_empty());
+        assert_eq!(status(&s, Stage::Loads), Optional);
+        assert_eq!(status(&s, Stage::Solve), Next, "{:?}", s.progress());
+    }
+
     #[test]
     fn model_check_returns_the_checklist() {
         let mut s = Session::default();

crates/model/src/bcs.rs

@@ -172,6 +172,16 @@ impl BoundaryCondition {
         }
     }
 
+    /// Whether it drives the model on its own, as a load does: a non-zero prescribed
+    /// displacement or a non-zero prescribed temperature (a support only holds).
+    pub fn drives(&self) -> bool {
+        match self {
+            BoundaryCondition::Displacement { values, .. } => values.iter().flatten().any(|v| *v != 0.0),
+            BoundaryCondition::Temperature { value, .. } => *value != 0.0,
+            BoundaryCondition::Fixed { .. } | BoundaryCondition::Symmetry { .. } => false,
+        }
+    }
+
     /// Whether it prescribes temperatures (rather than motion).
     pub fn is_thermal(&self) -> bool {
         matches!(self, BoundaryCondition::Temperature { .. })