engine: one scene builder for the window and headless pictures

engine::view::build (Style, Show, Palette) is the viewport's builder,
moved from ui-egui; ui-egui maps theme::Tokens onto a Palette and calls
it. Palette::LIGHT now equals the light theme's tokens. Test: the app's
scene and engine::view::frame's are identical for the same session.

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

crates/ui-egui/src/theme.rs

@@ -6,6 +6,7 @@
 //! ATTRIBUTION.md. Widget states are coloured with craft-ui's `WidgetPalette`.
 
 use egui::{Color32, Stroke};
+use fieldcraft_engine::view::Palette;
 use fieldcraft_model::ElementKind;
 use fieldcraft_render::Rgb;
 use fieldcraft_render::glyphs::GlyphColors;
@@ -257,6 +258,28 @@ impl Tokens {
         }
     }
 
+    /// The viewport's colours for the scene builder ([`fieldcraft_engine::view::build`]).
+    pub fn palette(&self) -> Palette {
+        Palette {
+            background: (rgb(self.viewport_top), rgb(self.viewport_bottom)),
+            hex: rgb(self.hex),
+            tet: rgb(self.tet),
+            wedge: rgb(self.wedge),
+            shell: rgb(self.shell),
+            line_element: rgb(self.line_element),
+            mesh_edge: rgb(self.mesh_edge),
+            node: rgb(self.node),
+            geometry: rgb(self.geometry),
+            geometry_edge: rgb(self.geometry_edge),
+            bodies: self.bodies.map(rgb),
+            no_result: rgb(self.no_result),
+            contour: self.contour_stops(),
+            contour_over: rgb(self.contour_over),
+            marked: rgb(self.error),
+            glyphs: self.glyph_colors(),
+        }
+    }
+
     /// The colour elements of this kind are drawn in.
     pub fn element(&self, kind: ElementKind) -> Color32 {
         match kind {