feat: Convert to Sheet Metal takes a solid or a shelled body by its bend edges (#725)

Convert to Sheet Metal now takes a solid or a shelled body by its bend edges (`bends`): the faces reached from the fixed face across those edges become the sheet, keeping the part's envelope, with the flat pattern by the rule's K-factor and radius. Other edges between sheet faces are cut open. The result works with edge flanges, cuts, Unfold/Refold, the flat pattern and DXF. A conversion without `bends` is the plate conversion as before. The optional `bends` field is noted under format 17. Fixes #647.

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

docs/book/src/guide/files.md

@@ -58,7 +58,7 @@ is available. Missing or incompatible pictures use the usual thumbnail generatio
 The default `cache: false` omits the portable thumbnail too.
 The repository's `docs/design-container.md` describes limits and encoding details.
 
-Current designs use format 17, including face anchors on a piece of a face by its own name, sheet metal rims and bend ends, physical materials defined in the design, the font of sketch text, sheet metal corner relief shapes, sketch face anchors on cylinder caps, sketch axis
+Current designs use format 17, including face anchors on a piece of a face by its own name, sheet metal rims and bend ends, bend edges for Convert to Sheet Metal, physical materials defined in the design, the font of sketch text, 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