calendar: give Advent's ordinary ferias their own precedence tier

Found while starting the December sanctoral pull: Advent's plain
(non-Ember) weekday ferias have real standing in the live Monastic 1617
engine that the existing ordinary-feria category couldn't represent --
a Simplex saint (St. Bibiana, Dec 2) is merely commemorated there rather
than winning outright, while a Semiduplex+ saint (St. Nicholas, Dec 6)
wins with the feria itself commemorated back. Adds a third, weaker
privileged-feria-minor tier (Semiduplex+ threshold, vs. privileged-
feria's Duplex+) rather than reusing either existing tier, mirroring how
privileged-feria/privileged-feria-major were split in March. Lent's own
ordinary ferias likely share this property but are left as-is, unverified
this session.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/calendar/commemorations.ts

@@ -84,6 +84,18 @@ export function decideOccurrence(
       // feast, however low-ranked, is kept in its place.
       return { winner: sanctoralWinner(sanctoral), commemorations: [] };
 
+    case 'privileged-feria-minor':
+      // Advent's own ordinary ferias (verified: St. Bibiana, Simplex,
+      // merely commemorated under an Advent feria; St. Nicholas,
+      // Semiduplex, wins outright with the feria commemorated in return).
+      // Weaker than `privileged-feria`'s Duplex+ threshold — Semiduplex is
+      // already enough here — and unlike every other feria tier, nothing
+      // ever transfers: whichever side loses still gets a commemoration.
+      if (isAtLeast(sanctoral.rank, 'semiduplex')) {
+        return { winner: sanctoralWinner(sanctoral), commemorations: [{ kind: 'temporal', id: temporalId }] };
+      }
+      return { winner: temporalWinner, commemorations: [sanctoralCommemoration(sanctoral)] };
+
     case 'privileged-feria':
       // The lesser of the two privileged-feria tiers (Ember days outside
       // Holy Week/the Easter Octave, etc.) — behaves exactly like an