Reprocess FEATS: relax filter to |f_z|<0.4 (was 0.5) + keep 1.5% background frames for VAE diversity → 16,969 rows total (81 bg, 0.5%)

Backfill domain='real' on feats parquet

feats: remove 5302 no-contact frames (|f_z|<0.5 N), keep 16711

feats: add gel_variant column (black_dot vs different gel)

add feats parquet shards