Add Our Lady of Guadalupe (Dec 12)

Sourced from the reference engine's national-calendar file. Own proper
collect, Ant 1, and Matins Nocturn 2 narrative reading (Juan Diego's
apparition account), same scope as the Our Lady of Lourdes feast --
Common of the BVM (C11) carries the rest. No canonization-deflation
question here (an apparition title, cultus confirmed 1754/1910). At
Duplex she ties the Immaculate Conception octave's threshold on a
non-closing day, which favors the occurring saint, so she wins Dec 12
outright.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EwsZQMjALCvy7u9tFDWmuQ

src/data/calendar/sanctoral-calendar.yml

@@ -469,6 +469,7 @@ days:
   "12-09": [st-juan-diego]
   "12-10": [st-melchiades]
   "12-11": [st-damasus]
+  "12-12": [our-lady-of-guadalupe]
   "12-13": [st-lucy]
   "12-15": [st-eusebius-of-vercelli]
   "12-20": [vigil-of-st-thomas]