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/ss-nereus-achilleus-domitilla-and-pancras.yml

@@ -0,0 +1,8 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617), live kalendar
+# check (2029-05-12, no Sunday collision) -- winner outright, Semiduplex.
+# Own proper collect, no unique antiphon of their own.
+id: ss-nereus-achilleus-domitilla-and-pancras
+name: "Ss. Nereus, Achilleus, Domitilla, and Pancras, Martyrs"
+rank: semiduplex
+common: common-of-several-martyrs
+propers: "ss-nereus-achilleus-domitilla-and-pancras"