Flat-pattern DXF for a laser shop: layers by role, dashed bend lines, bend notes, a file per body (#1324)

Closes #1284. sheet.export_dxf writes layers by role (OUTLINE, CUT, TEXT or ETCH, dashed BEND_UP/BEND_DOWN, optional BEND_NOTES, POCKET_<side>_Z…_MM and PERFORATION), inner contours before the outline; text: cut|etch, bend_notes and note_height, per_body files. Letters are derived from the timeline, nothing new is saved (format unchanged). I merged main in (the export's parameter list keeps main's pocket layer names). Checked on main + this branch: cargo check --workspace, the engine cmd::sheet tests (160) and the io DXF tests.

crates/io/src/lib.rs

@@ -26,7 +26,7 @@ pub mod urdf;
 pub mod vfs;
 
 pub use dxf::{read_dxf, write_dxf};
-pub use dxf_out::{DxfEntity, DxfLayer, DxfShape, loop_shapes, seg_shape, write_dxf_drawing};
+pub use dxf_out::{DxfEntity, DxfLayer, DxfLinetype, DxfShape, loop_shapes, seg_shape, write_dxf_drawing};
 pub use legacy_dir::{APP_ID, LEGACY_APP_ID, adopt_legacy_dir, legacy_flatpak_path};
 pub use safe_write::{backup_path, write_atomic};
 pub use sketch2d::{Geom2, MAX_DRAWING_BYTES};