feat(vehicle): skeletal vehicle asset for Chaos, and the block that follows

Chaos drives a skeletal mesh, so the static system split was not drivable on its own. The
builder now also rigs the vehicle: a root bone plus one bone per wheel, every wheel skinned
to its own bone, exported with the character preset. SK_Vehicle_Sedan imports with
SK_Vehicle_Sedan_Skeleton (508 triangles).

Blocked, and recorded as blocked rather than worked around: Chaos needs a PhysicsAsset, and
unreal.PhysicsAssetFactory in 5.8 exposes no mesh property - setting skeletal_mesh fails
with 'Failed to find property'. Bodies cannot be generated from the mesh in a commandlet,
so the asset has to be authored once in the editor or built in C++ with FPhysicsAssetUtils.

Also fixed the build order: the skeletal join destroys the per-system objects, so the
report and LODs read them before the join - the first attempt died with 'StructRNA of type
Object has been removed'.

Tools/ue_vehicle_import.py

@@ -102,6 +102,33 @@ def main():
         mesh = unreal.EditorAssetLibrary.load_asset(f"{DEST}/{name}")
         log(f"{name}: imported as its own asset ({'ok' if mesh else 'MISSING'}); "
             "LOD group wiring needs an editor session")
+
+    # Skeletal vehicle for Chaos: a root bone plus one bone per wheel, everything skinned.
+    skel_path = os.path.join(SOURCE, "vehicle_skeletal.fbx")
+    if os.path.exists(skel_path):
+        task = unreal.AssetImportTask()
+        task.filename = skel_path
+        task.destination_path = DEST
+        task.destination_name = "SK_Vehicle_Sedan"
+        task.automated = True
+        task.save = True
+        task.replace_existing = True
+        ui = unreal.FbxImportUI()
+        ui.import_mesh = True
+        ui.import_as_skeletal = True
+        ui.import_animations = False
+        ui.import_materials = True
+        ui.skeletal_mesh_import_data.import_uniform_scale = 1.0
+        task.options = ui
+        unreal.AssetToolsHelpers.get_asset_tools().import_asset_tasks([task])
+        skel = unreal.EditorAssetLibrary.load_asset(f"{DEST}/SK_Vehicle_Sedan")
+        if skel:
+            skeleton = skel.get_editor_property("skeleton")
+            # Skeleton has no bone_names property in 5.8; read the reference skeleton
+            # through the asset registry instead of guessing a property name.
+            log(f"SK_Vehicle_Sedan: imported, skeleton={skeleton.get_name() if skeleton else 'NONE'}")
+        else:
+            log("SK_Vehicle_Sedan: MISSING after import")
     lines.append("VERDICT: PASS")