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/app/update/mod.rs

@@ -1426,6 +1426,7 @@ impl OpenCADStudio {
                 Task::none()
             }
             message @ (Message::AttachPick
+            | Message::UnderlayAttachPick(_)
             | Message::AttachPickResult(_)
             | Message::XrefAttach(_)
             | Message::XrefAttachBrowseResult(_)) => self.update_xref_attach(message),