feat(ef): the Mass preface, RG 482-499

Item 2 of Phase 3 (celebrant-rubrics-phase1), the bulk of this phase
and the last EF Mass rubric this project scoped: which preface is
said, deferring to seasonal/proper-title rules for the fourteen named
prefaces, then RG 498's Common residual.

RG 482 gives the resolution chain: the Mass's own proper preface,
failing that the seasonal one, failing that Common. Read literally,
RG 484-497 look like fourteen separate rules, but each numbered
rubric's own propria/de-Tempore pair produces the SAME preface
identity either way, so the whole chain collapses into one priority-
ordered decision: title/mystery triggers (Holy Cross, Sacred Heart,
Christ the King, Trinity, St Joseph, BVM, the Nativity octave, the
Apostles, Epiphany), each independent of season, then six seasonal
windows (Nativity, Epiphany, Lent, Holy Cross/Passiontide, Easter,
Ascension, Holy Spirit, Trinity-for-ordinary-Sundays), then Common.

The "one genuinely unproven piece" the design spec worried about --
per-feast proper prefaces extracted from the Missal's propers at scale
-- turned out not to be a large-scale extraction problem at all: every
one of the fourteen propers is a closed, small, subject/slug-keyed
trigger (mirroring Precedence_ef.band's own RG 91 table), not
thousands of individual saints' pages. The one genuine open question
(RG 488, the Chrism Mass) is N/A: this engine resolves one Mass per
civil day and has no separate Chrism-Mass dimension.

Preface is a new kernel type (lib/kernel/preface.ml[i]), not an
EF-specific one, the same placement as Colour/Subject/Mass_formulary:
Liturgical_day.t is parameterised only over season/rank, so any field
it carries generically must live in the kernel even though only EF
constructs a value of it today. Wired exactly as creed/gloria were
(Rite.t.preface, Calendar.ml, Rite_ef.context), but Preface.t option,
not a bare bool: unlike creed/gloria, a preface is said only at a
Mass, and Good Friday (1955-restored Holy Week) resolves an observed
celebration but has no Mass at all -- None is the honest answer there,
and also the neutral value an unimplemented rite returns.

Two priority-order findings only the oracle settled, not derivable
from the Latin text in isolation, both cross-checked against 358
individually classified entries in the FIUV Ordo's own praef column
(test/fixtures/fiuv-ordo-2025-2026.sexp, already captured, wired up
here as a new comparison axis): RG 484(b)'s own "except Masses with a
proper of the divine mysteries or Persons" is narrower than every
other window's implicit exception (an Apostle inside the Nativity
octave is overridden to Nativity; outside it, keeps his own preface
even inside another window); and RG 495's "et votivis" half is live
for the one office this engine models without a votive-Mass dimension
(the Saturday Office of the BVM), which also proved a vigil is not a
"festum" for this purpose (the Assumption's own vigil takes Common,
not BVM, correcting an initial reuse of Precedence_ef.marian_slugs
that had no reason to make that distinction for its own, different
rubric). A third, RG16(a)-shaped fix landed the same way: RG 494(b)'s
own Trinity grant must read the day's TEMPORAL season, not the
celebration that actually won it, or a Class1 feast with no preference
of its own (All Saints) wrongly falls to Common on a Sunday it merely
commemorates.

colitur rubrics gains a sixth TAB-separated column. Domain-wide
1583-9999 exhaustive sweep confirms every Christmastide day resolves
Nativity/Epiphany/Bvm, every Paschaltide day one of its own three
windows or a season-independent title, every Lent day Lent or a
title, and Passiontide legitimately produces Easter exactly once a
year (the Vigil Mass) -- 8416 of the domain's 8417 years, the one
short year being the domain's own upper boundary (the liturgical year
opening in 9999 cannot construct dates in year 10000, a pre-existing
edge this sweep re-confirms rather than a new one).

day/readings verified byte-identical to the branch's own state before
this phase (commit 9c96e0a) across a 455-year sample spanning the
whole domain -- not literally to the v0.10.1 tag, which 22 earlier
commits on this same branch (Phase 1/2, the bissextile shift, the
Rogation colour fix) had already moved past before this phase began.

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