feat(data): ship a worked example overlay calendar

`--overlay` shipped last week with nothing to point at. The only example
was a test fixture, explicitly labelled invented and living where no user
would find it, so "colitur supports local calendars" was true but
undemonstrated.

data/ef/examples/diocesan-example.sexp is installed to
<prefix>/share/colitur/examples/ and is runnable documentation: every
celebration in it is invented, with example- slugs that cannot collide,
and its header explains the four directives, the three date shapes, and
the field vocabularies before showing any of them.

It demonstrates the range deliberately rather than the minimum: a
fixed-date local patron with its own propers; a movable dedication on the
first Sunday of October, I class because a church's own dedication
anniversary is I class in that church and at III it would lose to the
Sunday every year; an Easter-relative commemoration; an Edit raising a
universal feast for local use; and a Suppress shown commented out,
because suppressing a universal feast is a real liturgical act and an
example should not invite it casually.

Two things the header says plainly, since this is the one file that lets
someone change what colitur computes: an overlay is applied ON TOP of the
shipped calendar and never instead of it, and an overlay is applied, NOT
validated -- the five test layers assert things about the shipped data
and cannot vouch for a user's file.

A test asserts it loads against the real EF rank vocabulary, still
demonstrates at least four directives, and applies with NO diagnostics.
The last matters most: a directive naming a slug the universal calendar
lacks would warn, and an example that warns teaches the wrong lesson. It
deliberately does not pin which days it produces -- those celebrations
are invented, and pinning their dates would make an illustrative file
behave like calendar data.

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