quickstart: recommend snapshot_download so downloads register

fix stale v4 references in quickstart/citation; corrected physics-fusion figures

SpeedNet v5: corrected 790 m indoor supervision, multi-seed selection, per-mode evaluation, four GPS-verified demo videos

tags telemetry/karting + motivation section + demo embed

add tags: imu, gps, dji, gopro

SpeedNet v4 release: weights, self-contained inference, full docs, examples