rename findlib_index to findlib_index.json, standardise meta_files key
Writers (jtw.ml opam mode): output filename changed from findlib_index
to findlib_index.json, JSON key from "metas" to "meta_files".
Reader (findlibish.ml): universe link resolution uses .json extension.
Runtime default (impl.ml): changed from "findlib_index" to
"findlib_index.json".
JS client, test fixtures, HTML test pages, and documentation updated
to match.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>