docs: fix round 1 ride-along findings (F2-F7)

F2: test_christmastide_feria_slugs's 26 December assertions removed --
St Stephen/John/the Holy Innocents (RG 67-70) always outrank a Class4
Saturday candidate there, in every year, so the office can never
actually be observed on any day in the 26-28 December stretch. 3
January (no competing sanctoral entry, any year) used for the "-1-"
stretch demonstration instead, alongside the pre-existing 10 January
one.

F3: test_differential.ml's shared row type gains a subject field
(populated for colitur's own rows only; lectio's fixture has no such
column). C17's own predicate now also requires subject = "bvm" --
defence in depth against a future bug producing a white Class4
Saturday candidate some other way, particularly in Christmastide/
Paschaltide where colour alone could not catch it.

F4: Precedence_ef.band's entry-27 comment gains its RG 78 citation,
replacing "per the historical default that fills it".

F5: two report inaccuracies corrected -- band was already receiving a
real candidate 312,966 times pre-change (what was missing was its
identity, not its reality); the "Missae de S. Maria in sabbato" heading
quote was presented as "both scans, word for word" when both scans are
OCR-noisy there AND genuinely differ in abbreviation from each other --
corrected in temporal_ef.ml's own comment and the register alongside
the report.

F6: the colour rule gains a tighter citation chain -- RG 431(e) (the
Missal's own classification of this Mass as a "Missa votiva IV
classis... de B. Maria Virg.") -> RG 121(a) (votive Masses take the
colour of the feast-type they correspond to) -> RG 120(b) (BVM feasts
are white) -- reaching the same conclusion through the category (votive)
the Missal's own text actually places this Mass in, not by treating it
as a festum RG 91's own table structure says it is not.

F7: the oracle's real seasonal coverage recorded precisely in CLAUDE.md
and the register -- of 26 BVM-Saturday-office days in the 2026-2027
fixture only 17 are colour-discriminating; Time after Epiphany (5,015
observed days domain-wide) has zero oracle witnesses.

Full account in .superpowers/sdd/2026-08-12-colitur-rg16a/
bvm-saturday-report.md (local, gitignored).

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