import: IMPORT window and DGNIMPORT converting DGN models to drawing objects

The DGN V7/V8 readers expose each element (level, symbology, geometry,
complex chains, text nodes, shared cells). DGNIMPORT / -DGNIMPORT convert a
model with the Standard mapping into a new drawing; IMPORT opens the
Import File window and routes PDF and DGN files to their importers.

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

src/command.rs

@@ -2269,6 +2269,8 @@ pub enum CmdResult {
     SetTexteditMode(bool),
     /// Return a resolved point back to a suspended parent command (e.g. MTP / M2P).
     ReturnPoint(DVec3),
+    /// Import a DGN model (DGNIMPORT); ends the command.
+    DgnImport(crate::modules::insert::dgn_import::DgnImportRequest),
 }
 
 /// What kind of value the active command is currently asking for. Drives