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_raster.rs

@@ -214,8 +214,11 @@ pub fn page_size_inches(path: &str, page: &str) -> Option<(f64, f64)> {
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub struct PageAdjust {
     /// 0..=100; 100 keeps the colours, lower values pull their lightness
-    /// towards one third.
+    /// towards `contrast_pivot` (hue and saturation kept).
     pub contrast: u8,
+    /// The lightness contrast 0 leaves every colour at, in thousandths:
+    /// measured 500 for PDF, 243 for DWF and 650 for DGN underlays.
+    pub contrast_pivot: u16,
     pub monochrome: bool,
     /// Lighten dark, unsaturated content on a dark background (black lines
     /// and text become white), keeping coloured content.
@@ -266,11 +269,12 @@ pub fn adjusted_pixels(path: &str, page: &str, raster: &PdfPage, adjust: PageAdj
             rgb = hsl_to_rgb(h, s, (1.0 - l + 20.0 / 255.0).min(1.0));
         }
         if adjust.contrast < 100 {
-            // Lightness is pulled towards one third, hue and saturation kept:
-            // contrast 0 leaves every colour at a third of full lightness.
+            // Lightness is pulled towards the pivot, hue and saturation kept:
+            // contrast 0 leaves every colour at the pivot's lightness.
             let (h, s, l) = rgb_to_hsl(rgb);
             let k = adjust.contrast as f32 / 100.0;
-            rgb = hsl_to_rgb(h, s, 1.0 / 3.0 + (l - 1.0 / 3.0) * k);
+            let pivot = adjust.contrast_pivot as f32 / 1000.0;
+            rgb = hsl_to_rgb(h, s, pivot + (l - pivot) * k);
         }
         for (dst, c) in px[..3].iter_mut().zip(rgb) {
             *dst = (c.clamp(0.0, 1.0) * 255.0).round() as u8;