test(temporal_ef): fix round 1 (F5) -- the anchor erosion test proved nothing

test_holy_family_anchor_present_and_erosion_is_caught built its
"eroded" list by filtering the exact pair it then asserted was
absent -- removing an element and checking it is gone proves
List.filter works, not that any detector caught anything. Borrowed
the name of test_anchors_erosion_is_caught, which uses a genuine
one: missing_from_anchors, called with an "expected" set sourced
independently of T.anchors (named_slugs_for_year, via T.named).

Reworked to match that shape: the expected slug is now sourced from
T.sunday_slug (season/weekday logic, a different function entirely
from T.anchors's own hand-typed string), and both the presence and
erosion checks reuse the real missing_from_anchors detector instead
of a bespoke, self-referential one.

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