fix(of): correct vigil_feast's justification for of-nativity-vigil

The prior comment claimed of-nativity-vigil is 'not a distinct celebration' -
wrong: Temporal_of.named (temporal_of.ml:230) constructs it as its own
Sollemnitas-rank candidate on 24 December, a genuine contestant in
band/Precedence.resolve.

The real reason None is safe is structural, not that. The Nativity is
Tabula entry 2 (band 20); only entry 1, the Paschal Triduum (band 10),
bands lower. The Triduum's Easter-2..Easter window is confined to 20
March-25 April because Gregorian Easter never falls outside [22 March, 25
April] over the whole 1583-9999 domain (measured exhaustively in
test_computus.ml, not assumed), so it can never reach 25 December. The
Nativity therefore always keeps its own day and its vigil can never be
orphaned by a transfer or reduction.

Behaviour is unchanged: vigil_feast still returns None unconditionally.
RG 33 (EF-only) and Normae nn. 11/13 remain the reason the hook exists at
all and returns None for every other candidate.

1 files changed, 12 insertions, 3 deletions

diff --git a/lib/rites/rite_of/precedence_of.mli b/lib/rites/rite_of/precedence_of.mli
index 6b30dad..e81d35e 100644
--- a/lib/rites/rite_of/precedence_of.mli
+++ b/lib/rites/rite_of/precedence_of.mli
@@ -69,9 +69,18 @@ val admit :
   (Vocab_of.rank Precedence.candidate * Precedence.privilege) list
 
 (** [vigil_feast]: always [None]. RG 33's third omission trigger is EF-only;
-    the OF has no vigil OFFICE that its feast's movement could orphan. The
-    Nativity's "of-nativity-vigil" is a Mass formulary of the solemnity, not a
-    separate celebration. *)
+    the OF has no vigil OFFICE that its feast's movement could orphan --
+    Normae nn. 11/13 create no such independent celebration for anything
+    other than "of-nativity-vigil". THAT candidate is not exempt by lacking
+    standing: {!Rite_of.Temporal_of.named} constructs it as its own
+    Sollemnitas-rank candidate on 24 December, a genuine contestant in
+    {!band}/{!Colitur_kernel.Precedence.resolve}. It is exempt structurally
+    instead -- the Nativity (Tabula entry 2, band 20) can only ever be beaten
+    by the Triduum (entry 1, band 10), whose window (Easter-2..Easter) is
+    confined to 20 March-25 April because Gregorian Easter never falls outside
+    [22 March, 25 April] (measured exhaustively, 1583-9999, not assumed), so
+    it can never reach 25 December. The Nativity therefore always keeps its
+    own day, and its vigil can never be orphaned. *)
 val vigil_feast : Vocab_of.rank Precedence.candidate -> Slug.t option
 
 (** The four functions assembled for {!Colitur_kernel.Precedence.resolve}. *)