engine: animate a fluid step's frames

results.animate refused fluid steps because nothing deforms. A fluid step
instead plays its frames over time, the start-up transient from the field at
rest to the settled flow, the way an explicit step plays its frames. The
animation clock maps across the fluid frames once per cycle, and the fluid
results panel gains an Animate control beside its frame slider.

Part of #164.

crates/engine/src/cmd/results.rs

@@ -44,7 +44,7 @@ pub const MODES: CommandSpec = CommandSpec {
 pub const ANIMATE: CommandSpec = CommandSpec {
     id: "results.animate",
     label: "Animate",
-    params: r#"{"on": true} — animates the deformed shape: a mode swings through its positive and negative shape, a static step grows from undeformed to deformed"#,
+    params: r#"{"on": true} — animates the result over time: a mode swings through its positive and negative shape, a static step grows from undeformed to deformed, an explicit or fluid step plays its frames in order"#,
     form: Some(super::forms::ANIMATE),
     kind: CommandKind::Session,
     requires: &[Requirement::Result],
@@ -341,12 +341,12 @@ fn animate(s: &mut Session, p: &Value) -> Result<Value> {
         return Ok(s.results_report());
     }
     let step = step_for(s, &json!({}), Want::Any)?;
-    if s.latest_result(&step).is_some_and(|(r, _)| matches!(r.kind(), Kind::Fluid(_))) {
-        return Err(EngineError::Params(format!("step `{step}` is a fluid step: nothing deforms; show its frames with results.frame")));
-    }
+    let fluid = s.latest_result(&step).is_some_and(|(r, _)| matches!(r.kind(), Kind::Fluid(_)));
     let v = view_of(s, &step)?;
     v.animate = true;
-    v.deformed = true;
+    // A fluid step has nothing to deform; animating plays its frames (as results.frame does) in
+    // order. A structural step animates its deformation (or an explicit step its frames).
+    v.deformed = !fluid;
     Ok(s.results_report())
 }
 
@@ -1484,7 +1484,10 @@ mod tests {
         assert_eq!((r["traced"]["seeds"].as_u64(), r["traced"]["points"].as_u64()), (Some(5), Some(20)), "{r}");
         assert!(s.execute("results.streamlines", &json!({"show": false})).unwrap()["streamlines"].is_null());
         assert_eq!(s.execute("results.surface", &json!({"show": false})).unwrap()["surface"], false);
-        assert!(s.execute("results.animate", &json!({})).is_err(), "nothing deforms");
+        // A fluid step animates its frames (the start-up transient), not a deformation.
+        let r = s.execute("results.animate", &json!({})).unwrap();
+        assert_eq!((r["view"]["animate"].as_bool(), r["view"]["deformed"].as_bool()), (Some(true), Some(false)), "{r}");
+        assert!(s.execute("results.animate", &json!({"on": false})).unwrap()["view"]["animate"] == json!(false));
         assert_eq!((s.journal.len(), s.revision()), (journal, revision), "display commands are not edits");
         // Structural steps have none of it.
         let mut t = solved();