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/propers/nocturn-readings/nativity-of-st-john-the-baptist.yml

@@ -13,7 +13,13 @@
 # per the pre-1955 pass's convention of not transcribing bare incipits
 # — only the homily portion is authored (not marked isGospel),
 # combined with Lectio8-9 as one continuous excerpt of St. Ambrose's
-# commentary. No responsory given for either reading.
+# commentary.
+#
+# Both reading entries below carry their closing proper responsory:
+# Nocturn 2's is [Responsory6] (the last of Lectio4-6); Nocturn 3's is
+# [Responsory8] (the last of Lectio7-9 that has one — [Responsory9]
+# doesn't exist, since Te Deum is sung after Lectio9 at this
+# Duplex-1-classis rank).
 id: nativity-of-st-john-the-baptist
 readings:
   - nocturn: 2
@@ -85,6 +91,9 @@ readings:
         Lord. And this is why John said of himself: I am the voice of one crying in the
         wilderness, Make straight the way of the Lord.
     status: { la: verified, en: verified }
+    responsory:
+      la: "R. Innuébant patri ejus quem vellet vocári eum: et póstulans pugillárem, scripsit dicens: * Joánnes est nomen ejus.\nV. Apértum est os Zacharíæ, et prophetávit dicens.\nR. Joánnes est nomen ejus."
+      en: "R. They made signs to his father, how he would have him called; and he asked for a writing-table, and wrote, saying: * His name is John.\nV. The mouth of Zacharias was opened, and he prophesied, saying:\nR. His name is John."
 
   - nocturn: 3
     isGospel: false
@@ -151,3 +160,6 @@ readings:
         had not heard, since it had been revealed to her by the same Holy Ghost Who had commanded
         the Angel to tell it.
     status: { la: verified, en: verified }
+    responsory:
+      la: "R. Gábriel Angelus appáruit Zacharíæ dicens: Nascétur tibi fílius, nomen ejus Joánnes vocábitur: * Et multi in nativitáte ejus gaudébunt.\nV. Erit enim magnus coram Dómino, vinum et síceram non bibet.\nR. Et multi in nativitáte ejus gaudébunt."
+      en: "R. The Angel Gabriel appeared unto Zacharias, and said thy wife Elizabeth shall bear thee a son, and thou shalt call his name John; * And many shall rejoice at his birth.\nV. For he shall be great in the sight of the Lord, and shall drink neither wine nor strong drink.\nR. And many shall rejoice at his birth."