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/locale_catalog.rs

@@ -2985,7 +2985,7 @@ pub(super) fn message_attribute(source: &str) -> Option<(&'static str, &'static
         "Page size:" => Some(("insert", "pdfui-page-size")),
         "Reference name:" => Some(("insert", "pdfui-reference-name")),
         "Search for layer" => Some(("insert", "pdfui-search-for-layer")),
-        "This file does not contain any layers." => Some(("insert", "pdfui-this-file-does-not-contain-any-layers")),
+        "This file does not contain any layers" => Some(("insert", "pdfui-this-file-does-not-contain-any-layers")),
         "Select an underlay to view its layers." => Some(("insert", "pdfui-select-an-underlay-to-view-its-layers")),
         "PDF data to import" => Some(("insert", "pdfui-pdf-data-to-import")),
         "Vector geometry" => Some(("insert", "pdfui-vector-geometry")),