feat(ef): RG 111(a), the sung-Mass commemoration cap

Item 1 of Phase 3 (celebrant-rubrics-phase1): the "at Low Mass"
commemoration-placement rule the design spec recorded as unread. It is
not a new rule -- it is the sung/low axis of RG 111, which colitur
already implements.

RG 111(a) (LT.txt, "Ratio admittendi commemorationes"): a liturgical
day of the first class, AND any non-conventual sung Mass regardless of
the day's own class, admits at most one commemoration, and only if it
is privileged. (b)/(c)/(d), the same rubric's remaining clauses, give
the wider caps colitur's admit already computes -- which is exactly
the LOW MASS answer.

Exposed as Precedence.sung_mass_commemorations, a pure derivation over
the existing Low-Mass admitted list (filter to Privileged, keep the
first), not a new stored field on Liturgical_day.t: the input list is
already validated and privilege-tagged, so a second field would only
create a second place for the same fact to drift out of sync with the
first, with no new information gained. Liturgical_day.t.commemorations
is now documented as the Low Mass set explicitly, removing the
ambiguity its .mli previously left unstated.

Tested against synthetic Low-Mass sets (none/one/two privileged,
already-first, empty) and two real calendar days resolved through the
normal Cal.day pipeline: 2026-08-14 (Vigil of the Assumption, an
ordinary-only commemoration, dropped at Sung Mass) and 2026-04-25 (the
Major Litanies, RG 80/109(f), privileged, kept at both Masses).

0 files changed, 0 insertions, 0 deletions