DGN underlays: global origin, stored extents, colour table display, 3D cells

- V8 models are placed relative to their global origin (model header).
- A V8 model's extent is the union of its elements' stored ranges; a V7
  model's text box is one text height tall.
- Files without a colour table use the standard DGN default table; colour
  255 is read from the table's background entry.
- Colours from a file's colour table follow the background: near-white
  colours swap with it, and over non-black backgrounds some shades drop a
  step.
- DGN contrast pulls towards a pivot taken from the model's own colours,
  after the fade, which is blended into the background first.
- Shared cells are read through their full 3D placement, so tilted
  instances and 3D definitions are drawn in plan.
- Deleted V8 elements are skipped; lines along the model's extent stay in
  the raster.

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

src/scene/model/underlay_vector/dwf.rs

@@ -180,7 +180,7 @@ pub fn sheet(bytes: &[u8], name: &str) -> Option<Sheet> {
         xps::read(data, w2x.map(|v| v.as_slice()), entry, &family)?
     };
     let rect = view.or_else(|| paths_bounds(&paths))?;
-    Some(Sheet { rect, paths, texts, sub_per_master: 1.0 })
+    Some(Sheet { rect, paths, texts, sub_per_master: 1.0, table_colors: false, element_colors: Vec::new() })
 }
 
 /// An XPS obfuscated font (.odttf): its first 32 bytes are XORed with the