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/locale_catalog.rs

@@ -2983,6 +2983,25 @@ pub(super) fn message_attribute(source: &str) -> Option<(&'static str, &'static
         "Import PDF" => Some(("insert", "pdfui-import-pdf")),
         "Select one or more pages from the PDF file:" => Some(("insert", "pdfui-select-one-or-more-pages-from-the-pdf-file")),
         "Page size:" => Some(("insert", "pdfui-page-size")),
+        "Attach Underlay" => Some(("insert", "ul-attach-underlay")),
+        "Attach DWF" => Some(("insert", "ul-attach-dwf")),
+        "Attach DGN" => Some(("insert", "ul-attach-dgn")),
+        "Attach DWF Underlay" => Some(("insert", "ul-attach-dwf-underlay")),
+        "Attach DGN Underlay" => Some(("insert", "ul-attach-dgn-underlay")),
+        "{count} sheets" => Some(("insert", "ul-sheet-count")),
+        "{count} models" => Some(("insert", "ul-model-count")),
+        "Sheets" => Some(("insert", "ul-sheets")),
+        "Models" => Some(("insert", "ul-models")),
+        "Selected sheet: {sheet}" => Some(("insert", "ul-selected-sheet")),
+        "Selected model: {model}" => Some(("insert", "ul-selected-model")),
+        "Sheet size:" => Some(("insert", "ul-sheet-size")),
+        "Model size:" => Some(("insert", "ul-model-size")),
+        "Conversion units" => Some(("insert", "ul-conversion-units")),
+        "Master units" => Some(("insert", "ul-master-units")),
+        "Sub units" => Some(("insert", "ul-sub-units")),
+        "Sub units make the default scale {scale}" => Some(("insert", "ul-sub-units-scale")),
+        "Attaches a DWF or DWFx file as an underlay." => Some(("insert", "ul-attaches-dwf")),
+        "Attaches a DGN file as an underlay." => Some(("insert", "ul-attaches-dgn")),
         "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")),