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 <[email protected]>

crates/engine/src/cmd/features.rs

@@ -1532,3 +1532,7 @@ mod offset_edit_guard_tests;
 #[cfg(test)]
 #[path = "offset_plane_face_tests.rs"]
 mod offset_plane_face_tests;
+
+#[cfg(test)]
+#[path = "stacked_face_tests.rs"]
+mod stacked_face_tests;