feat(io): FBXEXPORT writes the drawing's 3D meshes as binary FBX

FBXEXPORT (also EXPORTFBX) saves every 3D mesh of the drawing (solids,
surfaces and meshes, at full display resolution) as a binary FBX 7.4
file for visualisation tools: one model per object with its geometry,
face normals and a material in its display colour. Axes stay Z-up and
the unit scale follows the drawing's insertion units.

Refs #1502

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

src/app/commands/display.rs

@@ -940,6 +940,11 @@ impl OpenCADStudio {
                 return Some(Task::done(Message::StlExport));
             }
 
+            // FBXEXPORT — export 3D meshes to FBX for visualisation tools
+            "FBXEXPORT" | "EXPORTFBX" => {
+                return Some(Task::done(Message::FbxExport));
+            }
+
             // STEPOUT — export 3D meshes to STEP AP203 format
             "STEPOUT" | "EXPORTSTEP" | "STPOUT" => {
                 return Some(Task::done(Message::StepExport));

src/app/commands/mod.rs

@@ -918,6 +918,7 @@ inventory::submit!(crate::command::CommandRegistration {
         "SHORTCUTS",
         "SSM",
         "STEPOUT",
+        "FBXEXPORT",
         "STLOUT",
         "STPOUT",
         "STYLE",