MovingCircle keeps the applied offset and an optional pending target apart,
validates targets against the applied offset (box, Euclidean 0.5-cell step,
speed, no sliding hold) and commits only after upload, step and sync succeed;
any failure after the upload attempt is fail-closed. Simulation.add_body
(translation=...) declares one; moving circles are excluded from the packed
static links, unsupported modes and swept-band placement are rejected at
initialize, and every positive-step run raises before any device write until
the moving kernels exist. Host tests include a refill-donor enumeration.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
