Author real proper Matins/Vespers content for the Nativity of St. John the Baptist

nativity-of-st-john-the-baptist.yml's `common` field (common-of-an-apostle)
was the same mistaken placeholder as the Beheading's, but this feast
needed a different fix: the reference engine assigns it no Commune at
all -- every [Rule] line in Sancti/06-24.txt and SanctiM/06-24.txt points
to proper text, never a Commune category -- so there's no better-fitting
id to swap in either.

Authored the real proper content that was still falling through to the
apostle Common, live-verified against the local Divinum Officium CGI
instance (Monastic Tridentinum 1617):

- Matins invitatory antiphon ("Regem Præcursóris Dóminum")
- Matins hymn ("Antra desérti téneris," with the Monastic doxology
  substitution already used by this feast's own Lauds/Vespers hymns)
- Full Matins Nocturn 1-3 psalmody + nocturn versicles (Nocturn 1/2 reuse
  St. Lawrence's own shared psalm-number pool per "Psalmi Dominica";
  Nocturn 3 reuses St. Lawrence's own canticle scripture files verbatim,
  since the citations are identical)
- Vespers psalmody (only 4 psalm slots at this Monastic rank, not 5 as a
  naive reading of the raw Ant Vespera list would suggest)
- Nocturn 2/3 responsories on the existing nocturn-readings file

`common: common-of-an-apostle` is left in place as a fully inert
placeholder (a non-optional field with no better id available) with an
updated comment explaining why.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WvnBMq7HGfXRyMsVLu2v4D

src/data/calendar/saints/nativity-of-st-john-the-baptist.yml

@@ -3,10 +3,19 @@
 # classis, the highest rank pulled in this set. About St. John the
 # Baptist's own nativity, not a feast of the Lord/BVM, so kept in the
 # sanctoral store like the Conversion of St. Paul/Chair-of-Peter feasts
-# despite the very high rank. Own proper collect and antiphon. "common-
-# of-an-apostle" is a loose fit (he isn't one) but the closest existing
-# category to "a major male saint honored with his own high feast" --
-# worth reconsidering if a more precise category becomes worth adding.
+# despite the very high rank. Own proper collect and antiphon.
+#
+# `common: common-of-an-apostle` is a placeholder, not a liturgical
+# claim -- the reference engine assigns this feast no Commune at all
+# (every [Rule] line in Sancti/06-24.txt and SanctiM/06-24.txt points to
+# proper text, never a Commune category), so no better-fitting id exists
+# either. Now verified-inert: every part this field could ever be
+# consulted for (Matins invitatory/hymn/Nocturn psalmody, Vespers
+# psalmody, the Lauds/Vespers bundle, Benedictus antiphon, collect, all
+# four minor hours) has its own real proper text authored, so this field
+# is never actually reached by any resolver (see
+# data/hours/matins-psalmody-overrides/saints/nativity-of-st-john-the-baptist.yml's
+# own comment).
 id: nativity-of-st-john-the-baptist
 name: "The Nativity of St. John the Baptist"
 rank: duplex-1-classis