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 <noreply@anthropic.com>

src/app/commands/mod.rs

@@ -834,6 +834,7 @@ inventory::submit!(crate::command::CommandRegistration {
         "SELECTIONCYCLING",
         "UCSDETECT",
         "SURFACEASSOCIATIVITY",
+        "DGNIMPORTMODE",
         "LAYOUTCREATEVIEWPORT",
         "TXT2MTXTCOMBINEMTEXT",
         "TXT2MTXTTEXTORDER",