DGNIMPORT: reference naming, cells, text placement, curves, references

Name clashes, DGNIMPORTMODE and prefixing as the reference names them;
V7 cell copies, unshared cells and mirrored placements as blocks;
justified text and text nodes placed at their justification points with
the reference's MTEXT contents; B-spline curves as splines and 3D arcs and
ellipses in their own planes; every V8 level as a layer; design models
only; reference attachments translated, ignored or attached as underlays.
The colour index palette comes from the codec fix it depends on.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/settings.rs

@@ -519,6 +519,8 @@ pub struct UserSettings {
     /// PDFSHXTEXT's recognition settings (fonts, threshold, PDFSHXLAYER,
     /// PDFSHXBESTFONT, PDFSHXTEXTSETTINGS).
     pub pdf_shx_text: crate::modules::insert::pdf_shx_text::ShxTextSettings,
+    /// DGNIMPORTMODE and the DGN Import window's last choices.
+    pub dgn_import: crate::modules::insert::dgn_import::DgnImportSettings,
     /// Nested-copy symbol handling: false inserts, true binds.
     pub ncopy_bind: bool,
     /// Last Options page; unknown saved names fall back without rejecting the config.
@@ -999,6 +1001,7 @@ impl Default for UserSettings {
             ucs_view: true,
             layout_create_viewport: true,
             pdf_shx_text: Default::default(),
+            dgn_import: Default::default(),
             ncopy_bind: false,
             cursor_type: CursorType::Crosshair,
             crosshair_color: None,