calendar: pull May's sanctoral calendar (pre-1955 pull, 5/12)

18 saints for May (16 real + 2 stubs). Both stubs this month are
genuinely unwinnable in the real historical calendar in any year (their
competing observances are fixed-date, not movable) — flagged explicitly
in their own files as distinct from the usual "just an artifact of the
year checked" caveat.

A second, distinct feast of St. Michael (his Apparition, May 8) is kept
as its own record separate from st-michael.yml's Sept 29 dedication
feast, same precedent as the two Chair-of-Peter feasts.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/data/calendar/saints/st-urban-i.yml

@@ -0,0 +1,14 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617), live kalendar
+# check (2038-05-25, clear of the movable Pentecost Ember Friday that
+# usually overshadows this date) -- winner outright, Simplex. Own proper
+# collect; his own [Ant 1] is just a cross-reference to the generic Common
+# template, so no unique antiphon is recorded. Being Simplex, this date
+# doesn't help settle whether Pentecost's own Ember days share Lent's
+# lesser privileged-feria tier or the stricter major one (see
+# temporal-categories.yml's open caveat) -- a higher-ranked saint would be
+# needed to test that, and none turned up this month.
+id: st-urban-i
+name: "St. Urban I, Pope and Martyr"
+rank: simplex
+common: common-of-a-martyr-bishop
+propers: "st-urban-i"