Nobody depends on this yet, which makes now the moment to commit to a surface
rather than later when someone does.
docs/versioning.md says what that surface is. A calendar has a wider one than
a library usually does: the JSON shape, the fasting level names and their
order, the jurisdiction keys, the command line, and the two language APIs are
all things a caller can reasonably build on, and only one of them is a
function signature. The text output, the reason strings and the menologion's
wording are explicitly not.
It also names the category semver has no word for. A correction to a fast rule
or a feast rank breaks no interface, so it is a patch; but it changes what the
program tells someone to eat. Those are called out in the changelog under
Calendar whatever the version does, and CHANGELOG.md starts by listing every
one of them against pre-1.0 behaviour.
daysToPascha is documented as signed, counting to the Pascha of that day's own
civil year, which it always was and nothing said.
