Surface Loft: sheets through open or closed sections (#847)

Surface Loft lofts open or closed curve chains, one section per sketch in pick order, into surface bodies. Adds the SurfaceLoft feature (format 18 note), the surface.loft command, a dialog with Edit Feature support, and kernel and engine tests.

Part of #757.

crates/kernel/src/lib.rs

@@ -117,7 +117,7 @@ pub use step_in::{ImportedBody, StepImport, StepNode, step_import, step_import_s
 pub use step_out::{ExportBody, ExportProduct, StepHeader, step_export_bodies, step_export_products};
 pub use surface_kind::{FaceSurface, face_surface};
 pub use surfaces::{
-    copy_faces, extend, extrude_sheet, patch_edges, patch_region, remove_faces, reverse_faces, revolve_sheet, stitch, sweep_sheet,
+    copy_faces, extend, extrude_sheet, loft_sheet, patch_edges, patch_region, remove_faces, reverse_faces, revolve_sheet, stitch, sweep_sheet,
     sweep_sheet_points, thicken, trim, unstitch,
 };
 pub use sweep_path::sweep_path;