feat: add Underlay entity support (PDF/DWF/DGN)

- New src/entities/underlay.rs: TruckConvertible (clip boundary polygon
  or cross marker), Grippable (insertion + boundary vertex grips),
  PropertyEditable (position, scale, rotation, contrast, fade, flags),
  Transformable (translate, mirror, scale, rotate)
- Wire EntityType::Underlay into all four match arms in traits.rs
- UNDERLAY command: FADE/CONTRAST/ON/OFF/CLIP ON|OFF/MONO ON|OFF
  subcommands to edit selected underlay entities
- Entity type name map entry for UNDERLAY

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/entities/mod.rs

@@ -16,6 +16,7 @@ pub mod mtext;
 pub mod point;
 pub mod polyline;
 pub mod raster_image;
+pub mod underlay;
 pub mod ray;
 pub mod solid;
 pub mod spline;