feat(ef): the Mass of Our Lady on Saturday (RG 309(a))

RG 78 gave the day Our Lady's office and its white colour; this gives it her
Mass. RG 309(a): "Missae de sancta Maria in sabbato ... iuxta temporum
diversitatem" -- the Missal prints five formularies and the season selects
among them.

Without it the day was kept as Our Lady's and then read the FERIA's Mass, so a
Saturday in Septuagesima announced Our Lady and read the parable of the
labourers. colitur hit and fixed the identical bug in its own v0.3.0.

The five, with the two seams that are easy to get wrong:
  Advent            Isa 7:10-15      / Luke 1:26-38
  Christmas Time    Titus 3:4-7      / Luke 2:15-20
  ...and on until the Purification (2 February), not merely to the end of
     Christmas Time
  thereafter        Ecclus 24:14-16  / Luke 11:27-28
  Paschaltide       Ecclus 24:14-16  / John 19:25-27  -- same first reading,
     different gospel, which is why the test asserts the gospel separately

Citations are colitur's, verified there against both photographic scans.

The Advent formulary is unreachable as the Saturday office -- Advent has no
IV-class ferias -- and is kept anyway, because RG 309's subject is the Masses
themselves and a missing branch would fall through silently if the rank rules
ever changed.

Guarded on the office rather than the day: a sanctoral feast that outranks the
Saturday keeps its own readings, so the observed slug must still be the
temporal one. All twelve BVM Saturdays of 2026 now agree with colitur on
readings, where none did before.

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