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/engine/src/cmd/sheet.rs

@@ -38,10 +38,14 @@ pub static COMMANDS: &[CommandSpec] = &[
         .at("SHEET METAL", "CREATE")
         .icon("convert_sheet")
         .params(
-            "body; face: [x,y,z] on its large flat face (a plate of even thickness), or that face's name from model.faces; rule?; \
+            "body; face: [x,y,z] on its large flat face (a plate of even thickness; recesses in one face up to half the thickness deep, \
+             such as an engraved label, become pockets of the sheet), or that face's name from model.faces; rule?; \
              bends?: [[x,y,z] on sharp edges] (a solid or a shelled body: `face` is the fixed face and the flat faces reached from it \
              across these edges become the sheet; a shell keeps its thickness, a solid takes the rule's inward; \
-             edges between two of those faces that are no bends are cut open with the rule's gap)",
+             edges between two of those faces that are no bends are cut open with the rule's gap; \
+             a point in the middle of a chamfer between two faces, or on a bend modelled with its radius (a cylinder between \
+             two flat faces tangent to it, either skin), makes it one bend of the rule's radius at their virtual sharp; \
+             the faces' holes become cut-outs and their engravings, up to half the thickness deep, pockets)",
         ),
     CommandSpec::new("sheet.manage_rules", "Sheet Metal Rules", rules).at("SHEET METAL", "MODIFY").icon("sheet_rules").params(
         "name (new or existing); thickness?, k_factor?, bend_radius?, relief_width?, relief_depth?, corner_relief?, hem_gap?, gap? (expressions; `Thickness` is the rule's; \
@@ -60,7 +64,7 @@ pub static COMMANDS: &[CommandSpec] = &[
         ),
     CommandSpec::new("sheet.export_dxf", "Export Flat Pattern as DXF", export_dxf)
         .noundo()
-        .params("body?, path?, curves?: exact|arcs (default exact: an ellipse as ELLIPSE, splines, text outlines and other Béziers as SPLINE; arcs: tangent arcs and lines within 0.001 mm, for machines that read only those) → DXF (AutoCAD 2000, mm): OUTLINE, BEND_UP and BEND_DOWN (centre lines) layers"),
+        .params("body?, path?, curves?: exact|arcs (default exact: an ellipse as ELLIPSE, splines, text outlines and other Béziers as SPLINE; arcs: tangent arcs and lines within 0.001 mm, for machines that read only those) → DXF (AutoCAD 2000, mm): OUTLINE, BEND_UP and BEND_DOWN (centre lines) layers, and pockets on POCKET_<BOTTOM|TOP|INTERNAL>_Z<from>_TO_<to>_MM"),
     CommandSpec::new("sheet.export_bend_table", "Export Bend Table", export_bend_table)
         .noundo()
         .params("body?, format?: csv|json (default csv), path? → bend table in mm and degrees, including rule and flat centre-line coordinates; a bend that cut-outs split has a row for each piece"),