During Export, we were accidentally duplicating the `velocity` attribute
data. Once inside the `write_surface_velocity` function (which was
correct) and again while writing out all "custom" attributes inside
`write_custom_data` (which was incorrect). Fixed by excluding the
"velocity" attribute inside `write_custom_data`.
During Import, we were only loading back in those "custom" primvars so
things happened to work, by accident, but only for USD files produced by
Blender. Now we import just the Velocities attribute which should work
with all files.
This should fully address #96182
Pull Request: https://projects.blender.org/blender/blender/pulls/120771
37cc387fcb