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.

2 files changed, 37 insertions, 2 deletions

diff --git a/lib/rites/rite_of/temporal_of.ml b/lib/rites/rite_of/temporal_of.ml
index 9aa898e..e375350 100644
--- a/lib/rites/rite_of/temporal_of.ml
+++ b/lib/rites/rite_of/temporal_of.ml
@@ -238,7 +238,33 @@ let named d =
   let easter = Computus.gregorian_easter y in
   let off n = Date.add_days easter n in
   let m = Date.month d and dd = Date.day d in
-  if m = 12 && dd = 24 then Some (Advent, "of-nativity-vigil", Colour.White, Sollemnitas)
+  if m = 12 && dd = 24 && Date.weekday d <> Date.Sun then
+    Some (Advent, "of-nativity-vigil", Colour.White, Sollemnitas)
+    (* NOT unconditional. Tabula I.2 ("Dominicae Adventus, Quadragesimae et
+       Paschae") outranks Tabula I.3 (solemnities generally, where the Vigil
+       Mass of the Nativity itself bands, per Precedence_of.band's own entry-3
+       branch) -- Normae n. 5 states it directly for exactly this class of
+       day: a Sunday of Advent/Lent/Easter yields to nothing but a still
+       higher-banked day. The Fourth Sunday of Advent is one of those
+       privileged Sundays every time 24 December falls on one.
+
+       The Vigil Mass is an evening Mass of the FOLLOWING solemnity (the
+       Nativity, 25 December), not an office belonging to 24 December's own
+       civil day -- the identical granularity reasoning
+       Precedence_of.band's own entry-1 comment already applies to Holy
+       Thursday (the Triduum opens with an evening Mass inside Holy
+       Thursday's civil day, so the day itself stays at entry 2, not entry
+       1) and that data/of/expected-divergences-litcal.sexp's L1 entry
+       applies on the season axis. At colitur's one-office-per-civil-day
+       granularity, only one identity can hold 24 December, and on a Sunday
+       that identity is the Fourth Sunday of Advent, not the Vigil -- so
+       [named] must return [None] here and let [temporal]'s dispatch fall
+       through to [sunday_slug], exactly as it already does for every other
+       Sunday [named] does not claim.
+
+       Every non-Sunday 24 December is unaffected: the Vigil still wins
+       there, as a plain late-Advent feria (Tabula II.9) is never privileged
+       against it. *)
   else if m = 12 && dd = 25 then Some (Christmas, "of-nativity", Colour.White, Sollemnitas)
   else if m = 1 && dd = 1 then Some (Christmas, "of-mary-mother-of-god", Colour.White, Sollemnitas)
   else if m = 1 && dd = 6 then Some (Christmas, "of-epiphany", Colour.White, Sollemnitas)
diff --git a/lib/rites/rite_of/temporal_of.mli b/lib/rites/rite_of/temporal_of.mli
index 982ff05..d2acadd 100644
--- a/lib/rites/rite_of/temporal_of.mli
+++ b/lib/rites/rite_of/temporal_of.mli
@@ -103,7 +103,16 @@ val christ_the_king : int -> Date.t
     threading extra state through every other branch too -- the exact
     reason EF's own [named] excludes its structurally identical Holy
     Family/Holy Name. {!temporal} applies all three as its own targeted
-    dispatch instead. *)
+    dispatch instead.
+
+    24 December is ALSO conditional, not unconditional like the rest of this
+    function's fixed dates: it yields [of-nativity-vigil] only when that date
+    is not itself a Sunday. On a Sunday, [None] is returned so {!temporal}'s
+    dispatch falls through to {!sunday_slug}'s own [of-advent-sunday-4] --
+    Tabula I.2/Normae n. 5's Sundays of Advent outrank the Vigil (Tabula
+    I.3), the same evening-Mass-inside-the-civil-day reasoning already
+    applied to Holy Thursday (see [named]'s own implementation comment for
+    the full argument). *)
 val named : Date.t -> (Vocab_of.season * string * Colour.t * Vocab_of.rank) option
 
 (** Week within the season, 1-indexed, Sunday-aligned (constant across a