IMPORT: 3D Studio files

A .3ds file opens the 3D Studio Import window: the file's objects with
their kind and material count, filtered by kind or name, checked one by
one or all at once, where to put them on layers and what a mesh with
several materials becomes. Checked meshes come in as polyface meshes on
the chosen layers with materials made from the file's colours, cameras
as named views; a mesh with several materials is split, given its first
material, given none, or asked about on the command line. Automation
drives the window with tds_import_dialog.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/command.rs

@@ -2305,6 +2305,8 @@ pub enum CmdResult {
     ReturnPoint(DVec3),
     /// Import a DGN model (DGNIMPORT); ends the command.
     DgnImport(crate::modules::insert::dgn_import::DgnImportRequest),
+    /// Import a 3D Studio file with every question answered; ends the command.
+    TdsImport(crate::modules::insert::tds_import::TdsImportRequest),
 }
 
 /// What kind of value the active command is currently asking for. Drives