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/st-marcellus-i.yml

@@ -73,6 +73,6 @@ readings:
         the authority of the Roman Pontiff. He ordained at Rome in the month of December
         twenty-five Priests, two Deacons, and twenty-one Bishops for diverse Sees.
     responsory:
-      la: "Dómine, prævenísti eum in benedictiónibus dulcédinis: * Posuísti in cápite ejus corónam de lápide pretióso.\nV. Vitam pétiit a te, et tribuísti ei longitúdinem diérum in sæculum sæculi.\nR. Posuísti in cápite ejus corónam de lápide pretióso."
-      en: "O Lord, thou hast prevented him with blessings of sweetness: * Thou hast set on his head a crown of precious stones.\nV. He asked life of thee, and thou hast given him length of days for ever and ever.\nR. Thou hast set on his head a crown of precious stones."
+      la: "R. Dómine, prævenísti eum in benedictiónibus dulcédinis: * Posuísti in cápite ejus corónam de lápide pretióso.\nV. Vitam pétiit a te, et tribuísti ei longitúdinem diérum in sæculum sæculi.\nR. Posuísti in cápite ejus corónam de lápide pretióso."
+      en: "R. O Lord, thou hast prevented him with blessings of sweetness: * Thou hast set on his head a crown of precious stones.\nV. He asked life of thee, and thou hast given him length of days for ever and ever.\nR. Thou hast set on his head a crown of precious stones."
     status: { la: verified, en: verified }