fix: faces of an extrude drawn over a face's projected edges keep an exact source (#505)

A profile traced over a face sketch's projected boundary left the extrude's caps and sides without an exact planar source, so a sketch or offset plane on the new top face was refused. The drawn curve is now the edge's source where it covers a projection. Clearer fixed-plane message; regression tests in the engine, UI harness and design corpus, plus a corpus-wide flat-face attachment check (known gaps under #491-#495).

Fixes #489

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/dialogs.rs

@@ -2321,7 +2321,7 @@ pub fn start_sketch(app: &mut SolveApp, sel: &Sel) -> Result<(), String> {
         _ => return Err("Select an origin plane, construction plane or planar body face".into()),
     };
     let before = app.cam;
-    app.run("sketch.create", json!({ "plane": plane }))?;
+    app.run("sketch.create", json!({ "plane": plane })).map_err(|error| crate::friendly_error("sketch.create", &error))?;
     app.pre_sketch_cam = Some(before);
     look_at_sketch(app);
     // On a face: look at the picked point, not the plane's origin (a corner of the face),