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/ui-egui/src/lib.rs

@@ -1115,7 +1115,7 @@ impl ModelApp {
     /// frame; `None` when no results are shown.
     fn plot_key(&self) -> Option<PlotKey> {
         let (view, result, current) = results::shown(&self.session)?;
-        let frame = results::frame(view, self.now);
+        let frame = results::frame(view, result, self.now);
         let pick = results::pick(view, result, frame);
         Some(PlotKey { revision: self.revision, result: Arc::as_ptr(result) as usize, current, view: view.clone(), pick, frame })
     }