fix(vehicle): author wheel axles horizontally; record reverse is not the axle

Wheel axles were vertical in Unreal (the FBX armature conversion rolled them 90 deg
about X). Fix: axis_forward:X in the character preset and a Blender-Z wheel tail so the
axle lands on Unreal Y. This is correct (reference pose FL roll 180 / FR roll 0) but
does not fix the reverse: -drivetest still -2737 cm/s, and FinalRatio sign is a no-op.
The remaining cause is the Chaos wheel spin-axis convention or engine torque sign.

Tools/unreal/ue_vehicle_import.py

@@ -119,6 +119,11 @@ def main():
         ui.import_animations = False
         ui.import_materials = True
         ui.skeletal_mesh_import_data.import_uniform_scale = 1.0
+        # The FBX is already authored in the Unreal frame (nose +X, axle along Y); the
+        # importer's default axis conversion otherwise rolls the armature 90 deg about X and
+        # the wheel axles land on Z, which drives the Chaos vehicle backward.
+        ui.skeletal_mesh_import_data.import_rotation = unreal.Rotator(0.0, 0.0, 0.0)
+        ui.skeletal_mesh_import_data.import_translation = unreal.Vector(0.0, 0.0, 0.0)
         task.options = ui
         unreal.AssetToolsHelpers.get_asset_tools().import_asset_tasks([task])
         skel = unreal.EditorAssetLibrary.load_asset(f"{DEST}/SK_Vehicle_Sedan")