calendar, precedence_ef: two more RG 91 gaps exposed by defect 4

Found during the mandated 2005-2050 before/after verification (not
one of the seven, but caused by defect 4's own fix, so fixed in the
same pass rather than left as a self-introduced regression). Neither
is a hypothetical: both are live in the fixed calendar and both were
individually confirmed via cmd/lectio-ef-dump before being written up
as tests.

1. RG 91 entry 4 (Immaculate Conception, Assumption BVM) sits ABOVE
   entry 6 (Sundays of Advent/Lent/Passiontide) -- unlike an ORDINARY
   I-class feast (entry 11, e.g. St Joseph), the Immaculate Conception
   (8 December) is not impeded by an Advent Sunday at all. Before
   defect 4, Advent Sundays were wrongly II class, so this was
   accidentally right (I beats II outright, no tie reached); once
   Sundays became I class the tie-break mattered for the first time,
   and precedenceEF had no branch for it -- the Sunday wrongly won.

2. RG 91 entry 5 (Vigil & Octave day of the Nativity) sits above entry
   6 the same way, and its own gap was worse than a wrong winner: the
   Vigil of Christmas (24 December) falling on Advent IV, once
   defect 4 made that a genuine I-class tie, sent the Vigil into
   transferIfImpededEF's forward walk -- which has no way to re-place
   a transfer crossing the Dec 31/Jan 1 boundary (celebrationDate
   re-resolves a fixed date using the YEAR OF THE DAY BEING QUERIED,
   so a walk landing in the following January can never match the
   query that produced it). The Vigil did not move to the wrong day;
   it vanished for the whole year, for every year 24 December is a
   Sunday (2006, 2017, 2023, 2028, 2034, 2045).

Both fixed the same way as the existing II-class-feast-of-the-Lord
bonus in precedenceEF (a one-line precedence adjustment keyed on
slug), generalised into a single beatsClass1Sunday helper covering
both RG 91 entries.

A separate, unrelated bug surfaced by the SAME verification pass and
fixed alongside it: transferIfImpededEF's destination check reused
precedenceEF's tie-break BAND to decide "is this day I or II class",
but band encodes a different question (which of two EQUAL-class
candidates wins a tie) -- an ordinary, non-Sunday II-class temporal
candidate YIELDS under defect 2's own fix (band 5), even though it is
genuinely II class. A day within the Octave of the Nativity (26-31
Dec, RG 67) is exactly such a day, so band<=3 alone let a transfer
wrongly land inside it (a real reproduction: "vigil-of-christmas"
would have landed on 29 December 2006 instead of vanishing outright,
caught while tracing the entry-5 bug above). Replaced with a direct
class test (isHighClass), which is both correct and simpler -- it no
longer needs the Sunday flag at all for this particular check.

Witnesses (precedence_ef_repro_test.go): TestImmaculateConceptionBeatsAdventSunday,
TestVigilOfChristmasSurvivesAdventSunday. Fail before this commit with:
  2013-12-08 observed = "ef-advent-sunday-2" want immaculate-conception-of-the-blessed-virgin-mary (RG 91 entry 4 beats entry 6)
  2006-12-24 observed = "ef-advent-sunday-4" want vigil-of-christmas (RG 91 entry 5 beats entry 6; must not vanish)
(full set: 2013/2019/2024 for the first, 2006/2017/2023/2028 for the second)

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