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>

apps/solidcraft-cli/tests/corpus/README.md

@@ -17,6 +17,10 @@ hostile variants of it never panic.
 | `modern-plastic` | bd98035 | shell, lip, boss, rest, snap fit, plastic rule |
 | `naming-split-pieces` | 9a979cf | format 13 names: a slot splits the top; fillet, chamfer and offset face name its pieces by number (`F2:top#2`) |
 | `split-body-pieces` | v0.3.2 (07e263d) | a slot along X cuts a block into two bodies; a fillet, a chamfer and an appearance refer to the pieces by body name (`Body1`, `Body1 (1)`) |
+| `issue-489-stacked-join` | v0.6.0 (809ecf7), desktop | a 70 × 84 extrude, then a 70 × 62 strip joined on its top face, sketched partly over the face's projected edges (issue #489; expected values from main at 2880525) |
 
-The scripts that made them are in `scripts/`. Never regenerate a file with a newer build: the
-point is that it was written by the old one. Add new files for new eras instead.
+The scripts that made them are in `scripts/` (`issue-489-stacked-join` was drawn in the desktop
+app and has none). Every flat face of every final body must also take an attached sketch and an
+offset plane; the faces that can't yet are listed with their issues in `design_corpus.rs`.
+
+Never regenerate a file with a newer build: the point is that it was written by the old one. Add new files for new eras instead.