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/modules/insert/mod.rs

@@ -53,7 +53,18 @@ impl CadModule for InsertModule {
                     title: "Reference",
                     tools: vec![
                         RibbonItem::LargeTool(xattach::tool()),
-                        RibbonItem::LargeTool(pdf_attach::tool()),
+                        // PDF, DWF or DGN; the face runs the last one chosen.
+                        RibbonItem::LargeDropdown {
+                            id: "UNDERLAY_ATTACH",
+                            label: "Attach Underlay",
+                            icon: pdf_attach::ICON,
+                            items: vec![
+                                ("PDFATTACH", "Attach PDF", pdf_attach::ICON),
+                                ("DWFATTACH", "Attach DWF", pdf_attach::ICON),
+                                ("DGNATTACH", "Attach DGN", pdf_attach::ICON),
+                            ],
+                            default: "PDFATTACH",
+                        },
                         RibbonItem::LargeTool(xclip::tool()),
                         RibbonItem::LargeTool(xadjust::tool()),
                         RibbonItem::LabeledTool(underlay_layers::tool()),