feat: corner reliefs at sheet flange corners, chosen by the rule (#488)

Sheet Metal Rules get a Corner Relief Shape (notch, round or rectangular), sized by the rule's corner relief, cut where two bends meet at a corner of the base. Notch, the default, keeps existing designs as they were. File format 16.

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

docs/book/src/guide/files.md

@@ -51,7 +51,7 @@ keeps the current design. Previews cannot be edited or measured. This experiment
 requires `preview: true` in an engine Save/Save As call; ordinary saves omit display caches.
 The repository's `docs/design-container.md` describes limits and encoding details.
 
-Current designs use format 15, including sketch face anchors on cylinder caps, sketch axis
+Current designs use format 16, including sheet metal corner relief shapes, sketch face anchors on cylinder caps, sketch axis
 links, persistent
 names from exact face history, exact planar shell-face anchors, persistent analytic offset boundary roles and
 association profile references, durable sketch face anchors, periodic fit splines and versioned