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/mod.rs

@@ -3851,6 +3851,8 @@ pub enum Message {
     XAttachPickResult(Result<std::path::PathBuf, String>),
     /// ATTACH: pick a drawing, image or PDF to reference.
     AttachPick,
+    /// DWFATTACH / DGNATTACH: pick the file (the result goes the ATTACH way).
+    UnderlayAttachPick(codec::entities::UnderlayType),
     /// The Reference slide-out's xref fading: amount dragged, drag done,
     /// switch.
     XrefFadeSlide(u8),