fix(of): Nativity Vigil no longer suppresses a Sunday of Advent

Temporal_of.named claimed 24 December unconditionally as
of-nativity-vigil (Sollemnitas), and temporal's dispatch matched named
before sunday_slug. On a Sunday 24 December, the Fourth Sunday of
Advent -- Tabula I.2, Normae n. 5's own privileged Sundays of Advent,
Lent and Easter -- was silently replaced by the Vigil.

named now returns None for 24 December when that date is itself a
Sunday, so temporal's dispatch falls through to sunday_slug and the
day resolves as of-advent-sunday-4. The Vigil Mass is an evening Mass
of the following solemnity, not an office belonging to 24 December's
own civil day -- the same civil-day-granularity reasoning already
applied to Holy Thursday (Precedence_of.band's entry-1 comment) and to
expected-divergences-litcal.sexp's L1 entry. Every non-Sunday
24 December is unaffected.

Audited every other named entry for the same latent shape (see
.superpowers/sdd/of-r8-advent-iv-report.md): none of them has it,
either because the date can never fall on a Sunday, or because the
date IS the definition of that Sunday (Palm Sunday, Easter, Pentecost,
Trinity, Christ the King), or because the competing movable-Sunday
windows are constructed to exclude that date.

Added test_nativity_vigil_yields_to_advent_sunday, covering
2028-12-24 and 2034-12-24 (both inside the litcal fixture window) and
the 2026-12-24 non-Sunday control. Re-ran the litcal layer: no
allow-list counts moved, since it compares only season and
Ordinary-Time week, and both readings of 24 December are "advent"
either way. Updated test_precedence_of.ml's "entry 3, not 9" comment,
which had carried this as a known, unfixed defect, to describe the
fix; its own pinned assertion (a hand-built Thursday candidate) needed
no change.

1 files changed, 23 insertions, 0 deletions

diff --git a/test/test_temporal_of.ml b/test/test_temporal_of.ml
index 7560413..1ccd098 100644
--- a/test/test_temporal_of.ml
+++ b/test/test_temporal_of.ml
@@ -230,6 +230,28 @@ let test_ferial_slugs () =
   Alcotest.(check string) "ordinary weekday" "of-ordinary-time-2-monday" (slug_of (d 2026 1 19));
   Alcotest.(check string) "lent weekday" "of-lent-6-monday" (slug_of (d 2026 3 30))
 
+(* R8 fix: a Sunday 24 December must yield the Fourth Sunday of Advent, not
+   the Vigil -- Tabula I.2/Normae n. 5, and temporal_of.ml's own [named]
+   comment for the full argument (the same civil-day-granularity reasoning
+   already applied to Holy Thursday). 2028-12-24 and 2034-12-24 are the two
+   such Sundays inside the litcal fixture window (2023-12-03..2035-12-01);
+   2026-12-24, a Thursday, is the non-Sunday control. *)
+let test_nativity_vigil_yields_to_advent_sunday () =
+  List.iter
+    (fun (y, label) ->
+      let dt = d y 12 24 in
+      Alcotest.(check bool) (label ^ ": 24 Dec is a Sunday") true (D.weekday dt = D.Sun);
+      Alcotest.(check string) (label ^ ": named claims nothing") "<none>" (named_slug dt);
+      Alcotest.(check string) (label ^ ": temporal resolves to Advent IV") "of-advent-sunday-4" (slug_of dt);
+      Alcotest.(check string) (label ^ ": season is still advent") "advent" (season_str dt);
+      Alcotest.(check (option int)) (label ^ ": week 4") (Some 4) (T.week dt))
+    [ (2028, "2028"); (2034, "2034") ];
+  (* Non-Sunday 24 December: unaffected, the Vigil still wins. *)
+  let thu = d 2026 12 24 in
+  Alcotest.(check bool) "2026: 24 Dec is a Thursday, not a Sunday" true (D.weekday thu <> D.Sun);
+  Alcotest.(check string) "2026: named still claims the vigil" "of-nativity-vigil" (named_slug thu);
+  Alcotest.(check string) "2026: temporal still resolves to the vigil" "of-nativity-vigil" (slug_of thu)
+
 let test_colours () =
   let colour_of dt = (T.temporal dt).Colitur_kernel.Temporal.office.Colitur_kernel.Celebration.colour in
   Alcotest.(check string) "Good Friday is red" "red" (Colitur_kernel.Colour.to_string (colour_of (d 2026 4 3)));
@@ -387,6 +409,7 @@ let suite =
       Alcotest.test_case "ordinary time resumption" `Quick test_ordinary_time_resumption;
       Alcotest.test_case "sunday slugs" `Quick test_sunday_slugs;
       Alcotest.test_case "ferial slugs" `Quick test_ferial_slugs;
+      Alcotest.test_case "nativity vigil yields to Advent Sunday IV" `Quick test_nativity_vigil_yields_to_advent_sunday;
       Alcotest.test_case "colours" `Quick test_colours;
       Alcotest.test_case "domain edges do not raise" `Quick test_domain_edges_do_not_raise;
       Alcotest.test_case "exhaustive domain sweep" `Slow test_exhaustive_domain_sweep ]