checkMovement worked out the position four times, nearly copy-pasted,
for lift down, lift up, tilt down and tilt up. All four now call
travelPosition(), which keeps the same arithmetic, including how the
elapsed time wraps with millis(). Tilt up with no tilt time still
stops the tilt direction in the caller.
transformPosition and transformCommand keep their behaviour through
the new displayPosition() and flipCommand() helpers. The native env
builds ShadeMotion.cpp so all of this can be unit tested on the PC.
No behaviour change.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
