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>

crates/doc/src/format.rs

@@ -81,6 +81,10 @@
 //!   bend (`rim.bend0`, `rim.bend1`). Version 16 designs need no transformation; the version
 //!   stamp makes 0.8.0 (which reads up to 16) ask for an update instead of meeting names it
 //!   can't resolve.
+//!   Convert to Sheet Metal can take bend edges (`bends` in the feature: points on the sharp
+//!   edges of a solid or shelled body that become bends). A conversion without `bends` is the
+//!   plate conversion it always was and saves as before; 0.8.0 would drop the bend edges and
+//!   refuse the body as no plate.
 
 use serde_json::Value;