Add St. Juan Diego (Dec 9)

Sourced from the reference engine's national-calendar file (canonized
2002, outside both Tridentine 1906 and Divino Afflatu 1954). Source
rank "Duplex optional" has no FeastClass slot; mapped to Duplex then
deflated one tier to Semiduplex per the late-canonization rule. Live-
verified he's actually a commemoration, not the day's winner: Dec 9
falls inside the Immaculate Conception octave, whose elevated
wins:duplex threshold outranks a plain Semiduplex.

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

src/data/calendar/sanctoral-calendar.yml

@@ -466,6 +466,7 @@ days:
   "12-06": [st-nicholas]
   "12-07": [st-ambrose]
   "12-08": [immaculate-conception]
+  "12-09": [st-juan-diego]
   "12-10": [st-melchiades]
   "12-11": [st-damasus]
   "12-13": [st-lucy]