Convert to Sheet Metal takes bent parts as modelled: radius and chamfer bends, holes (#1314)

A Convert to Sheet Metal bend point can now lie on a bend modelled with its radius (a cylinder tangent to two flat faces) or in the middle of a chamfer; either becomes one bend at the faces' virtual sharp, with the rule's radius. Holes and engravings carry over. Bend points on edges convert as before. Recorded under format 21. Tests in cmd::sheet::convert_tests; the sheet-metal guide is updated.

crates/doc/src/format.rs

@@ -184,6 +184,13 @@
 //!   extrudes save; or `legacy_per_side`). An extent without it keeps the per-side reading
 //!   designs always had, so their thickness and parameters don't change; 0.11.0 asks for an
 //!   update instead of doubling a total-distance extrude.
+//!   Under the same version, a Convert to Sheet Metal bend point can lie in the middle of a
+//!   chamfer (a flat strip between two faces), which then is one bend at the two faces' virtual
+//!   sharp; 0.11.0 bent the chamfer's nearest edge instead or found no edge there. Bend points
+//!   on edges convert as before.
+//!   A bend point can also lie on a bend modelled with its radius (a cylinder between two flat
+//!   faces tangent to it), which then is one bend at the two faces' virtual sharp with the
+//!   rule's radius; 0.11.0 found no edge there or refused the curved face.
 
 use serde_json::Value;