bible: embed only the Vulgate by default; other corpora opt-in

Split the bundled corpora so the default binary carries only the
public-domain Latin Vulgate. wuj/drb/grb move to corpora_optional/ and are
compiled in only with `-tags fullbible`, or dropped into the user corpora
dir at runtime. This keeps the distributed AGPL binary free of third-party
scripture text -- bible corpora are not covered by lectio's licence.

- bible: read corpus files/metadata from core + optional embed FS
  (embReadCorpus/embCorpusFiles); optionalFS is set only under the tag.
- config: default `versions` now reflects corpora actually available in the
  build (Vulgate first), so a vul-only binary offers no absent versions.
- Makefile: `make build` = vul only; `make build-full` embeds the rest;
  `make test` runs with -tags fullbible; check-corpora validates both dirs.
- tests: corpusmeta asserts on vul (the always-embedded corpus); the
  versions-default expectation follows availableVersions().

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