engine: geometry.faces.merge, faces meshed as one (virtual topology, #505)

geometry.faces.merge {name, faces} records a face merge in the model;
it is applied after the partitions whenever the design is opened or cut
again, undoes through the journal and is saved with the model. Meshers,
mesh.plan (features and rings see merge:<name>; a merged face is
prismatic when its B-rep faces are), checks and the viewport see one
face, drawn in the warm colour. geometry.faces.merge.delete removes it.

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

crates/ui-egui/src/lib.rs

@@ -1135,11 +1135,15 @@ impl ModelApp {
                 let marked = fieldcraft_engine::view::marked(&self.session);
                 fieldcraft_engine::view::mark(&mut s, &self.display.1, &marked, theme::rgb(t.error));
             }
-            // What Check Geometry found, in the error colour: its faces where geometry faces are
-            // drawn, short edges and a dot at each finding over everything.
+            // Faces merged for meshing, each one face in the warm colour where geometry faces are
+            // drawn; then what Check Geometry found, in the error colour: its faces, short edges
+            // and a dot at each finding over everything.
+            let faces = style != scene::Style::Wireframe && !(show.mesh && !self.display.1.tris.is_empty());
+            let bodies: Vec<&str> = self.pick.names.iter().map(|n| n.name.as_str()).collect();
+            if show.geometry && self.plot.1.is_none() && faces {
+                fieldcraft_engine::view::draw_merged_faces(&mut s, &self.display.1, &self.session, &bodies, theme::rgb(t.warm));
+            }
             if let Some(marks) = fieldcraft_engine::view::geometry_marks(&self.session).filter(|_| show.geometry && self.plot.1.is_none()) {
-                let faces = style != scene::Style::Wireframe && !(show.mesh && !self.display.1.tris.is_empty());
-                let bodies: Vec<&str> = self.pick.names.iter().map(|n| n.name.as_str()).collect();
                 fieldcraft_engine::view::draw_geometry_marks(&mut s, &self.display.1, &bodies, marks, faces, theme::rgb(t.error));
             }
             let mut datums = Aabb3::EMPTY;

crates/ui-egui/src/theme.rs

@@ -276,6 +276,7 @@ impl Tokens {
             contour: self.contour_stops(),
             contour_over: rgb(self.contour_over),
             marked: rgb(self.error),
+            merged: rgb(self.warm),
             glyphs: self.glyph_colors(),
         }
     }