engine: show temperature and wall heat in fluid results

Route saved temperature through slices, obstacle surfaces, streamlines and both result views, with availability and model units shared by commands and the UI. Show signed obstacle heat histories per complete face area at the selected frame, leaving unrecorded times uncoloured and retaining the saved slice settings.

Part of #157.

crates/engine/src/view.rs

@@ -511,7 +511,8 @@ fn picture(s: &Session, p: &Palette, section: Option<Section>, toward: Vec3) ->
         // The part as it is, with the flow drawn through and around it.
         let mut scene = model_scene(&base, p);
         let mut bounds = base.bounds;
-        if let Ok(fp) = crate::flow::plot(view, result, &s.traced) {
+        let names: Vec<&str> = bodies.iter().map(String::as_str).collect();
+        if let Ok(fp) = crate::flow::plot_with_surfaces(view, result, &s.traced, s, &base, &names) {
             if let Some(c) = &fp.contour {
                 let cell = |(k, v): (usize, f64)| (u32::try_from(k).unwrap_or(u32::MAX), v);
                 shown.contour = Some((c.field.id().to_string(), cell(c.min), cell(c.max)));