DWF/DGN underlays: clipping, layers, text, 3D DGN, colours and zoom extents

- PDFOSNAP, DWFOSNAP and DGNOSNAP are separate; UOSNAP sets them all and
  reads 2 while they differ. Enable Snap switches the selected kind's.
- DWFCLIP / DGNCLIP clip their own kind with the kind's prompts and message;
  CLIP routes DWF and DGN underlays there; Enter at a corner asks again.
- The right-edge underlay tools show for DWF and DGN too (import is PDF
  only).
- Underlay layers: DGN levels (V8 level table, "Default" listed as "0"; V7
  used levels named from the level name table), hidden through the layer
  override data with the DGN placeholder string first; a plotted DWF has
  none.
- V8 colour read from its colour word (by level, index, or a true colour
  drawn as its nearest index); 3D V8 elements drawn in plan (lines, line
  strings, ellipses and arcs by their orientation, text); a model's extent
  includes its text.
- DWF 6 TrueType text (font, height, rotation from the font opcode) and DWFx
  glyph runs are drawn; the default W2D colour map is the measured one.
- Contrast pulls lightness towards a per-kind pivot as measured (PDF 0.5,
  DWF 0.243, DGN 0.65).
- Zoom extents includes raster images and underlays whose frames are hidden.

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

src/modules/insert/xclip.rs

@@ -16,7 +16,6 @@
 // underlay as PDFCLIP, a raster image as IMAGECLIP, a layout viewport as
 // VPCLIP, anything else is "*Invalid selection*".
 
-use codec::entities::UnderlayType;
 use codec::types::Handle;
 use codec::EntityType;
 use glam::DVec3;
@@ -488,7 +487,7 @@ impl CadCommand for ClipCommand {
                 self.inner = Some(Box::new(XclipCommand::for_inserts(vec![handle], false)));
                 CmdResult::NeedPoint
             }
-            Some(EntityType::Underlay(u)) if u.underlay_type == UnderlayType::Pdf => {
+            Some(EntityType::Underlay(u)) => {
                 self.inner = Some(Box::new(PdfClipCommand::for_underlay(handle, u)));
                 CmdResult::NeedPoint
             }