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/scene/model/pdf_layers.rs

@@ -231,6 +231,9 @@ fn without_key(dict: &str, key: &str) -> Option<String> {
 /// them.
 pub const LAYER_OVERRIDE_APP: &str = "AdeskUnderlayLayerOverrideData";
 
+/// The first string of a DGN underlay's layer overrides, before the names.
+pub const DGN_OVERRIDE_PLACEHOLDER: &str = "???DGN_Placeholder???";
+
 pub fn hidden_layers(u: &codec::entities::Underlay) -> Vec<String> {
     u.common
         .extended_data
@@ -239,7 +242,7 @@ pub fn hidden_layers(u: &codec::entities::Underlay) -> Vec<String> {
         .filter(|r| r.application_name.eq_ignore_ascii_case(LAYER_OVERRIDE_APP))
         .flat_map(|r| r.values.iter())
         .filter_map(|v| match v {
-            codec::xdata::XDataValue::String(s) => Some(s.clone()),
+            codec::xdata::XDataValue::String(s) if s != DGN_OVERRIDE_PLACEHOLDER => Some(s.clone()),
             _ => None,
         })
         .collect()