DWF and DGN underlays: attach, display, snaps, frames and references

DWFATTACH / -DWFATTACH and DGNATTACH / -DGNATTACH attach a DWF / DWFx sheet
or a DGN (V7 or V8) model with the reference's prompts; ATTACH routes the
file types to them. The sheet or model is read into vector paths, drawn as a
transparent underlay raster (contrast, fade, monochrome and background
adjustment apply) and offers its geometry to object snaps. Definitions are
kept in ACAD_DWFDEFINITIONS / ACAD_DGNDEFINITIONS, the External References
palette lists and reloads them, and image textures get line-keeping mip
levels so thin underlay lines stay visible when zoomed out.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/io/xref_model.rs

@@ -303,7 +303,8 @@ pub fn wildcard_match(name: &str, pattern: &str) -> bool {
 pub enum RefKind {
     DwgXref,
     Image,
-    Pdf,
+    /// A PDF, DWF or DGN underlay.
+    Underlay,
 }
 
 /// Lifecycle state of a [`ReferenceEntry`].