Prepend R. to nocturn-readings responsories missing their opening marker

All 53 nocturn-readings files with a responsory field started straight
into the text with no leading R., unlike octave-readings' own R./V.
convention. Fixed in content per direct instruction (not injected at
render time), matching each file's own existing plain V./R. style rather
than switching to octave-readings' R./V. glyphs.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EWzE3kaSZCs79fPBrrE1aF

src/data/propers/nocturn-readings/post-pentecost-16.yml

@@ -66,6 +66,6 @@ readings:
         covetous man, who is an idolater, hath any inheritance in the kingdom of Christ and of God"
         (Eph. iv. 17, v. 5).
     responsory:
-      la: "Duo Séraphim clamábant alter ad álterum: * Sanctus, sanctus, sanctus Dóminus Deus Sábaoth: * Plena est omnis terra glória ejus.\nV. Tres sunt qui testimónium dant in cælo: Pater, Verbum, et Spíritus Sanctus: et hi tres unum sunt.\nR. Plena est omnis terra glória ejus."
-      en: "One Seraph cried unto another: * Holy, Holy, Holy is the Lord God of hosts; * The whole earth is full of His glory.\nV. There are Three That bear record in heaven: the Father, the Word, and the Holy Ghost, and these Three are One.\nR. The whole earth is full of His glory."
+      la: "R. Duo Séraphim clamábant alter ad álterum: * Sanctus, sanctus, sanctus Dóminus Deus Sábaoth: * Plena est omnis terra glória ejus.\nV. Tres sunt qui testimónium dant in cælo: Pater, Verbum, et Spíritus Sanctus: et hi tres unum sunt.\nR. Plena est omnis terra glória ejus."
+      en: "R. One Seraph cried unto another: * Holy, Holy, Holy is the Lord God of hosts; * The whole earth is full of His glory.\nV. There are Three That bear record in heaven: the Father, the Word, and the Holy Ghost, and these Three are One.\nR. The whole earth is full of His glory."
     status: { la: verified, en: verified }