kernel(validate): fold in Plan 2's carried guards

Three carried items from Plan 2's parked rulings, closed:

1. Slug uniqueness moves from a 200-sample QCheck property scoped to
   one rite (test_temporal_ef.ml) into Validate's own "slugs" check, so
   every consumer gets it. The resumed-Sunday exemption that property
   carried is dropped, not weakened elsewhere: Plan 2 verified zero
   duplicate slugs domain-wide (all 8 416 years), and by construction a
   resumed Sunday only ever backfills a week number Septuagesima cut
   short that same liturgical year, so it can never repeat a number
   that year's own January Sundays already used. The now-redundant
   property and its is_resumable_sunday_slug helper are removed from
   test_temporal_ef.ml; test_validate.ml's own domain-wide property
   covers the same ground for every consumer.

2. The anchors-erosion guard (Plan 2: deleting entries from a rite's
   anchors list left the whole suite green) is implemented, but not in
   Validate. Which of a rite's named days are Easter-derived is
   knowledge only the rite's own `named` function has; Rite.t
   deliberately exposes only `temporal` and `anchors`, never `named`,
   so a rite-agnostic Validate has no ground truth to check anchors'
   completeness against. Hardcoding an Easter offset, or even Easter
   itself, would smuggle Western/Gregorian-specific knowledge into code
   meant to also serve a future Julian-reckoning rite; rediscovering
   "named-ness" structurally from `temporal` alone is unsound for EF,
   since most ordinary Sunday/feria slugs from Septuagesima onward are
   also constant-offset-from-Easter by construction. The guard is
   therefore EF-specific and lives in test_temporal_ef.ml, discovering
   the Easter-derived slug set mechanically (scanning a window around
   Easter and keeping whatever `named` answers Some for) rather than
   hand-copying either named's or anchors' own offset list, then
   asserting completeness against the real anchors for the domain's
   Easter extremes (1598, 1666) plus an ordinary year. A negative
   fixture proves the guard has teeth, matching Plan 2's exact
   regression (anchors missing "ef-ascension" reports it, and only it,
   as missing).

3. test_validate.ml's extreme_years comment claimed 1818/2038; verified
   against Computus.gregorian_easter directly, the domain's actual
   Easter extremes (1583..2500) are 1598/1666. Corrected.

Verification: the full 1583..9999 domain sweep (233 tests via dune
test's 200-sample default, plus a manual full sweep) reports exactly
one failure -- the known, already-pinned year-9999 season-truncation
case -- and zero occurrences of the new "slugs" check anywhere in the
domain. Deleting "ef-ascension" from the real anchors list (reproducing
Plan 2's regression directly) is caught immediately by the new EF test
and, confirmed empirically, invisible to Validate's own full property
sweep -- direct evidence for why item 2 cannot live in Validate.

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