test(differential): C31 closed -- 4 classes left, 257 of 267 rows are naming

The ferias between Epiphany and the first Sunday after it repeat Epiphany's
own Mass in lectio now; they had been taking the Mass of the Sunday that
follows them.

  C31  98 -> 0
  C1   40 -> 138   grew, and changed character

C1 is now SLUG VOCABULARY ONLY, and its note says so. Season, rank, colour and
both citations agree with colitur on every day of the January window; what
differs is the identifier -- ef-christmas-2-thursday against
ef-time-after-epiphany-1-thursday for the same day with the same Mass. C6 is
the same shape.

That is 257 of the 267 remaining rows. Aligning the vocabularies would mean
renaming lectio's slugs, and those are keys: its own lectionary, clectio's
generated tables and any user overlay are built on them. Recorded as
vocabulary rather than closed, because the rows do differ -- just not in
anything a reader of either engine's output sees.

Measured for 2026: season, rank and colour agree on all 365 days; 14 days
differ and every one differs on the slug alone.

5 classes -> 4, agreement 98.4%.

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