Attach Underlay ribbon menu, DWF/DGN attach dialog, DGN true colours

- Insert > Reference: the PDF Attach button becomes an Attach Underlay menu
  (PDF / DWF / DGN); its face runs the last one chosen.
- DWFATTACH, DGNATTACH and ATTACH on a .dwf/.dwfx/.dgn open the Attach
  dialog for that kind: sheets or models with thumbnails (one chosen),
  placement, path type, file details with the sheet / model size, and for
  DGN the conversion units (sub units offer 1 / sub units per master as the
  scale).
- DGN V8 true colours come from the ExtendedColors record packed in the
  non-model attributes stream; the colour word's high part numbers them.
- A DGN model's extent includes each text's box (its length along the
  baseline and 1.5 x its height), as the reference sizes a model.
- DWFx text uses the family named in its embedded (deobfuscated) font.
- DWF contrast pivots at 0.73 once colours are turned over for a dark
  background.
- Hairline strokes are drawn solid one pixel wide, and underlay rasters are
  made at or below their size on screen so those lines stay unbroken.

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

src/scene/mod.rs

@@ -2753,7 +2753,7 @@ impl Scene {
         let Some(wpp) = self.world_per_pixel() else {
             return;
         };
-        let step = (f64::from(wpp).log2() * 4.0).floor() as i32;
+        let step = (f64::from(wpp).log2() * 4.0).ceil() as i32;
         let current = self.underlay_world_per_pixel.map(|w| (w.log2() * 4.0).round() as i32);
         if current == Some(step) {
             self.underlay_scale_pending = None;
@@ -2783,7 +2783,7 @@ impl Scene {
         let Some(wpp) = self.world_per_pixel() else {
             return false;
         };
-        let step = (f64::from(wpp).log2() * 4.0).floor() as i32;
+        let step = (f64::from(wpp).log2() * 4.0).ceil() as i32;
         self.underlay_world_per_pixel.map(|w| (w.log2() * 4.0).round() as i32) != Some(step)
     }