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

RG 78's office has been built since ef-bvm-saturday: an otherwise
unoccupied IV-class Saturday is de sancta Maria in sabbato, white, IV
class. The Mass stayed the feria's, so the day observed Our Lady and then
read Colossians on the parable of the weeds. That is M26 shape 1.

RG 309(a) is the rule -- the Masses the Missal assigns "iuxta temporum
diversitatem" -- and it prints five, each under its own season heading.
All ten citations are transcribed from the scans with line numbers in the
source. Masses II-V were confirmed twice, against the scan and against a
live missalemeum capture of 2038 which names them and agrees on both
readings. Mass I rests on the scan alone: no oracle year to hand has an
Advent Saturday carrying this office. One witness where the others have
two, and the code says so rather than blurring it.

The mapping looked like the hard part and dissolved. The Missal's ranges
are cut by civil dates and movable feasts, not by this rite's seasons --
II ends mid-season at the Purification, III spans four seasons. But
measured against what a IV-class Saturday can actually be, it reduces to
a seasonal mapping plus one date test at 2/3 February, with no Easter
arithmetic at all: III's end to IV's start leaves the Triduum unassigned,
unreachable because Holy Saturday is I class; IV's end to V's start
leaves Pentecost week unassigned, also unreachable because that week's
Saturday is an Ember Saturday. Time after Pentecost then begins exactly
at Trinity, exactly where Mass V begins. Both gaps verified by sweep --
zero IV-class Saturdays in either across 2000-2059 -- not inferred.

The office is identified structurally, never off the slug, which it
deliberately shares with the ordinary feria: subject Bvm, IV class,
Saturday.

Blast radius: 87611 reading rows over the full domain, every one a
Saturday.

Allow-lists: M26 27 to 6, shape 1 gone, leaving only the Christ-the-King
residue argued in colitur's favour last commit. The 2038 suite's
BVM-SATURDAY-MASS class is removed rather than zeroed -- an id with no
rows must not be declared. M18 355 to 376. C17 widened 397 to 445,
taking its own Saturdays back from C1, C19 and C25, since one cause
should not be filed under two ids; C31 89 to 103; C25 closed at 0 rows,
its Holy-Name-week Saturdays now answered before step 3 is reached.

Three unit tests pinned the old behaviour and are corrected rather than
re-pinned: two asserted the day "keeps its ferial Mass", which was the
honest thing to assert while this was unimplemented. The third still
tests what it always did -- that step 4 refuses a temporal office -- and
only what answers instead has changed.

Register: section 6.13.

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