engine: draw fluid results as slices, coloured obstacles and streamlines

After solve.fluid the results view had nothing to draw: a fluid step's
fields are per cell, the view only knew nodal fields, and the deformed
display it built dropped the geometry surfaces, so even the part
vanished.

A fluid step now has its own display (`engine::flow`, on render's
flow display data), shared by the headless pictures and the window:
the part drawn as it is, its faces coloured by the field in the fluid
beside them, the field on a slice through the domain and streamlines
seeded on the inlet face upstream of the obstacles, coloured by speed
when the legend shows velocity. It is first shown with velocity
magnitude on a slice through the middle, normal to the horizontal axis
across the flow.

New commands: results.slice {step, field: velocity|pressure, axis:
x|y|z|none, position, frame}, results.surface {step, show} and
results.streamlines {step, show, count, max_steps, seeds | line,
frame}, which replies with what it traced. results.contour takes
velocity and pressure for fluid steps (a new `pressure` field) with
the usual bands and legend clamp, and results.frame walks a fluid
step's frames. Streamlines leave the coming-soon list. Hostile
parameters are refused, and the counts are capped.

Part of #193.

crates/ui-egui/src/theme.rs

@@ -62,6 +62,9 @@ pub struct Tokens {
     pub line_element: Color32,
     /// Element edges (the mesh wireframe).
     pub mesh_edge: Color32,
+    /// Streamlines when the legend shows another field than velocity: stands out from the
+    /// viewport background.
+    pub streamline: Color32,
     pub node: Color32,
     // ---- geometry ----
     /// Faces of the open design and imported surfaces (SolidCraft's body colour).
@@ -149,6 +152,7 @@ impl Tokens {
             shell: Color32::from_rgb(0xc0, 0x8c, 0x78),
             line_element: Color32::from_rgb(0xc8, 0x8c, 0x1e),
             mesh_edge: Color32::from_rgb(0x1e, 0x23, 0x2a),
+            streamline: Color32::from_rgb(0x28, 0x2e, 0x38),
             node: Color32::from_rgb(0xb4, 0x3c, 0x8c),
             geometry: Color32::from_rgb(176, 186, 198),
             geometry_edge: Color32::from_rgb(40, 44, 52),
@@ -216,6 +220,7 @@ impl Tokens {
         t.shell = Color32::from_rgb(0xc8, 0x96, 0x82);
         t.line_element = Color32::from_rgb(0xf0, 0xc0, 0x50);
         t.mesh_edge = Color32::from_rgb(0x14, 0x18, 0x1e);
+        t.streamline = Color32::from_rgb(0xd8, 0xde, 0xe8);
         t.node = Color32::from_rgb(0xf2, 0xe6, 0x6e);
         t.geometry = Color32::from_rgb(152, 156, 162);
         t.geometry_edge = Color32::from_rgb(16, 16, 18);
@@ -272,6 +277,7 @@ impl Tokens {
             shell: rgb(self.shell),
             line_element: rgb(self.line_element),
             mesh_edge: rgb(self.mesh_edge),
+            streamline: rgb(self.streamline),
             node: rgb(self.node),
             geometry: rgb(self.geometry),
             geometry_edge: rgb(self.geometry_edge),
@@ -445,6 +451,20 @@ mod tests {
         }
     }
 
+    /// Streamlines in one colour (the legend showing pressure) stand out from the viewport
+    /// background in both themes (#193 review: they were near-black on the dark one).
+    #[test]
+    fn streamlines_stand_out_from_the_background() {
+        let luma = |c: Color32| i32::from(c.r()) * 3 + i32::from(c.g()) * 6 + i32::from(c.b());
+        for dark in [true, false] {
+            let t = Tokens::get(dark);
+            for bg in [t.viewport_top, t.viewport_bottom] {
+                assert!((luma(t.streamline) - luma(bg)).abs() > 1200, "dark={dark}");
+            }
+            assert_eq!(t.palette().streamline, rgb(t.streamline));
+        }
+    }
+
     /// The selection and hover colours differ from each other, from every element, geometry,
     /// node and symbol colour, and from the background.
     #[test]