calendar: RG 96 transfers must skip II-class days too (defect 1)

transferIfImpededEF's forward walk only skipped days whose temporal
office was I class. RG 96 requires skipping to "the next following
day that is not I or II class" -- II class also blocks. 2011: the
Sacred Heart (Friday after the Corpus Christi octave) falls on 1 July
and impedes the Precious Blood, also fixed on 1 July. The walk landed
the Precious Blood on 2 July, displacing the Visitation (II class,
fixed) outright, and never reached 3 July (an ordinary II-class
Sunday) at all.

Two changes were needed together, not one:

1. The walk's continuation threshold widens from "temporal band <= 2"
   to "<= 3", which is what actually captures every II-class temporal
   day (an ordinary Sunday, or a II-class named feast) as well as
   every I-class one -- band 3 is precedenceEF's own value for any
   non-Sunday-privileged II-class temporal candidate, unchanged by
   this commit.

2. A FIXED sanctoral II-class feast (the Visitation) has no temporal
   band at all -- the walk needs to also check whether some OTHER
   fixed celebration, class 1 or 2, already resolves onto the
   candidate day. computeEF now builds this check once (occupiedByRank,
   parameterised by which ranks count) and passes it in.

Widening (1) alone is wrong on its own: a class-1 feast is NEVER
actually impeded by a mere II-class day (I class always outranks II
class outright, no tie exists) -- naively applying the wide "<=3"
threshold to decide whether the ORIGINAL date is impeded, not just
where to land afterwards, wrongly bumped unimpeded feasts landing on
an ordinary Sunday (caught while testing this: All Saints, 1 Nov 2026,
a Sunday that year, was wrongly pushed to 3 Nov). transferIfImpededEF
now uses two different thresholds for two different questions -- class
1 only to decide IF a candidate is impeded at all, class 1 OR 2 to
decide where an already-impeded one may land -- see its own doc
comment for the full reasoning.

Witness (precedence_ef_repro_test.go): TestTransferSkipsBothIAndIIClass.
Fails before this commit with:
  2011-07-02 observed = "precious-blood-of-our-lord-jesus-christ" want
    visitation-of-the-blessed-virgin-mary (RG 96: the Precious Blood
    must skip past it, not displace it)
  2011-07-04 observed = "ef-time-after-pentecost-3-monday" want
    precious-blood-of-our-lord-jesus-christ (RG 96: first day that is
    neither I nor II class)

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