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 <[email protected]>

src/command.rs

@@ -1569,6 +1569,7 @@ pub enum CmdResult {
     /// page; the host creates or reuses the definitions and commits them all
     /// in one undo step, then ends the command.
     AttachPdfPages {
+        kind: codec::entities::UnderlayType,
         path: String,
         pages: Vec<(String, EntityType)>,
     },