rite(ef): pin down floor_div and week/sunday_slug agreement

Task 12 review: add a permanent regression test for floor_div's negative
branch (Ash Wednesday, 4 days before the Lent I origin -- the only date in
the system where it fires), and stop sunday_slug from independently
recomputing the Pentecost-relative week number that week already computes.
A cross-check test pins the two together before the refactor and continues
to guard against future duplication.

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