v0.12.1

The release that made the two READMEs match.

The split was already there, English in README.md and Japanese in README.ja.md, but
some sections existed in only one of them: the Japanese one had neither what the
invariants protect nor the licence. Of all the places to disagree, the front door is the
worst.

Changed

• The two READMEs line up again. The split is unchanged, English in README.md and
  Japanese in README.ja.md, but the sections that existed in only one are now in both:
  the Japanese one had neither what the invariants protect nor the licence.

  A stale number is gone too (403 tests). Both now say that scripts/check.sh is the
  one way in, and how to run the end-to-end suite and the seeder on their own.

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Changelog in Japanese
· Full Changelog: v0.12.0...v0.12.1