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]>

docs/face-attachments.md

@@ -6,12 +6,18 @@ and orientation are stored in the source face's canonical plane. Editing the sou
 or moving its placement therefore updates the sketch plane while preserving its local
 geometry and dimensions.
 
-Supported planar sources are exact Box faces, untapered New Body Extrude caps and straight
-sketch sides, and planar faces of immutable STEP imports. Extrude caps also identify their
+Supported planar sources are exact Box faces, untapered New Body and Join Extrude caps and
+straight sketch sides, and planar faces of immutable STEP imports. Extrude caps also identify their
 outer boundary and holes by source curve IDs. Reordering profiles cannot substitute another
-profile's cap. Curved surfaces cannot host a planar sketch through this workflow.
+profile's cap. A profile drawn over a face sketch's projected boundary (a rectangle
+traced over the whole face, or a strip whose sides lie on the face's edges) is identified by
+the drawn curves: where a drawn curve and a projected one cover the same edge, the drawn curve
+is the source. Two drawn curves, or two projections, over one edge remain ambiguous. Curved surfaces cannot host a planar sketch through this workflow.
 Edited surfaces without retained construction provenance, ambiguous sources, and unsupported
-generators require an explicit fixed plane instead. A nearby or parallel face is never used
+generators require an explicit fixed plane instead. A face that a Join merged from two features' coplanar
+faces belongs to neither, so it also needs a fixed plane. The error says why, and the Create
+Sketch dialog then offers **Use fixed plane**, which places the sketch on that face's current
+plane without following later changes. A nearby or parallel face is never used
 to repair a new attachment automatically.
 
 The flat faces of a sheet metal body are the exception: they have no construction