test: differential vs lectio 2005-2050 with a cited allow-list

Validation layer 3 (design spec's five): compares colitur's real day-by-day
EF output against lectio (sibling project, Go), 2005-2050, one line per
civil day. Of 16801 day-pairs, 11206 already agree on the seven leading
columns; the 5595 that don't resolve into exactly 25 distinct field-diff
signatures, all triaged.

Three strictly separate layers, per the controller's ruling (the brief's
single flat allow-list assumed a handful of differences, not 5595):

- Layer A (test_differential.ml, norm_season/norm_slug): vocabulary. An
  explicit, closed table of naming synonyms with no liturgical substance
  (lectio's easter/christmas vs colitur's paschaltide/christmastide; a
  handful of slugs that are two names for the identical office). No
  wildcards -- every entry is a literal string pair.

- Layer B (strip_epiphany_index): numbering. The one slug family whose
  lectio/colitur index offset is not a constant (Time-after-Epiphany week
  numbering, register 3c#5) has its embedded digit stripped to a common
  form on both sides before comparing; rank and colour stay fully compared.

- Layer C (data/ef/expected-divergences.sexp): the cited allow-list. Ten
  genuine liturgical disagreements, each citing its RG paragraph and
  naming which engine is right (always colitur, verified against the
  Missal/register, never against lectio's own behaviour). This is the
  only layer permitted to cover a difference in rank, colour, or which
  celebration is observed. Five extend or restate register 3c's already-
  documented divergences (season boundary, Sunday I-class, Advent Ember
  ferias, Rogations); five are new, found and adjudicated in this task
  (Lent Ember days, the Nativity Octave, Ember-day-vs-saint precedence,
  the St Joseph transfer off a Lent Sunday, and the 2011 Sacred Heart /
  Precious Blood / Visitation collision). expected_rows on each entry is
  an exact regression pin, asserted by the test, not documentation.

13 January (register 6's long-open "Baptism of the Lord" item) is
confirmed empirically fixed already -- Task 11's sanctoral wiring closed
it before this task started -- so it is not allow-listed; the only
residual difference there is the season boundary already covered by C1.

Two stated limits carried from the brief (commemorations are not
comparable; lectio's own EF oracle asserts season only, 2025-2026 only,
so a rank/colour difference is not presumptive evidence against colitur)
plus a third found during this task (the week column is a display
convention on both sides, not a liturgical fact, and is not compared at
all) are documented in the test file's own doc comment.

Fixture: test/fixtures/lectio-ef-2005-2050.txt, committed as plain text
(1.4 MB), generated by lectio commit 2386a45; provenance recorded in the
sibling .provenance file. Colitur's side is recomputed fresh from the
library on every run, through the same Calendar/Rite_ef pipeline
`colitur day` uses, not the compiled binary.

Proved the harness has teeth by two reverted perturbations: a genuine
colour difference injected into a fully-covered fixture row fails the
"no unexplained differences" check with the exact mismatched row printed;
a one-row drift in an allow-list entry's expected_rows fails the count
check independently, showing it is not merely a duplicate of the first
assertion. 236/236 tests green, clean-build verified, deterministic
across OCAMLRUNPARAM=R.

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