Add Vespers' ferial Preces, sourced from Tridentine 1906/1910

Monastic 1617 has no ferial Preces at Vespers, same gap Lauds already
had — the fuller "Preces feriales Vespera" is byte-for-byte the Lauds
ferial Preces with Psalm 129 swapped for Psalm 50 in the source, live-
verified against the same clean Advent feria already used for Lauds.
The short Sunday/feast form turns out to be identical at both hours too,
so it's reused directly rather than duplicated.

Factors the shared ferial-or-vigil predicate out of hours/lauds.ts into
resolve-common.ts's isFerialOrVigil so both hours' -preces cases resolve
it the same way instead of keeping two copies in sync by hand.

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

src/hours/sext.ts

@@ -70,6 +70,7 @@ function resolvePart(part: HourPart, day: LiturgicalDay): ResolvedPart[] {
     case 'vespers-psalmody':
     case 'vespers-office':
     case 'magnificat':
+    case 'vespers-preces':
       throw new Error(`Sext's ordo doesn't support a '${part.kind}' part`);
   }
 }