The download waits until the step's rebuild route is known; with the device
mask and device imposition the host rebuild reads and writes only the
pressure (the fresh cells' refill), so u, v, w and the old velocities stay on
the device (their upload wrote back the values just downloaded). Not under
RTX_E3_BAND_CHECK (its references read the whole mirror).
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>