feat: add glTF/GLB export tool to Gen mode

Add gen_export_gltf tool that serializes the current Bevy scene
(meshes, materials, transforms, hierarchy) into a binary .glb file
using the gltf-json crate.

Author: yiwang

Cargo.lock

@@ -28,3 +28,6 @@ clap = { version = "4.5", features = ["derive"] }
 # 3D generation via Bevy
 bevy = "0.15"
 image = "0.25"
+
+# glTF/GLB export
+gltf-json = { version = "1", features = ["names"] }

crates/gen/Cargo.toml

@@ -39,6 +39,9 @@ pub enum GenCommand {
         width: u32,
         height: u32,
     },
+    ExportGltf {
+        path: String,
+    },
 }
 
 // ---------------------------------------------------------------------------