stillwater-sc / universal / 34779940206

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fix(blockdecimal): store a two-digit DPD remainder as one 7-bit group (#1483)

DPD packs digits in 10-bit declets of three, with the one or two leftover digits at
the top. dpd_bits() reserves 4 bits for one leftover digit and 7 for two, but
blockdecimal stored two as a pair of 4-bit nibbles, 8 bits. Whenever
ndigits % 3 == 2 the high bit of the leading digit fell outside the storage, so a
leading 8 or 9 read back as 0 or 1: blockdecimal<8,DPD>(80000000) was 0,
dfixpnt<8,3,DPD>(82345.678) was 2345.678, and dfixpnt<8,3,DPD>::maxpos() was
19999.999 (#1477). On main, 40 of the 199 values of blockdecimal<2,DPD> and 40,000
of blockdecimal<5,DPD> fail to round-trip.

The two digits are now one DPD group: the declet of 0XY, whose upper three bits are
always zero, so it fits the 7 reserved bits. New dpd_encode_2digits() and
dpd_decode_2digits() in dfloat/dpd_codec.hpp do this; #1482 (dfloat drops the same
leftover digits entirely) can reuse them. The decoder reduces mod 100, so the 28
7-bit patterns the encoder never produces still give a two-digit value. For pairs
whose digits are both 7 or less, the group has the same bits as the old nibble
layout (all 64 such pairs checked), so only pairs containing an 8 or 9 get a new
bit pattern.

Tests:
- internal/blockdecimal/conversion/dpd.cpp (bd_dpd): the two-digit helpers for all
  100 values and all 128 patterns. Exhaustive round trips at 1, 2, 4 and 5 digits
  (6 to 8 at higher levels). Random digit strings written through setdigit in random
  order against BCD, plus maxval(), at every width from 1 to 12 and at 13 to 35
  digits at level 2.
- static/fixpnt/decimal/arithmetic/encodings.cpp: brings back the N % 3 == 2 widths
  (dfixpnt<8,4> Modulo and Saturate, dfixpnt<5,2>), which #1474's test had to skip,
  and checks that maxpos, maxneg and minpos agree across BCD, DPD and BID at every
  width from 1 to 12.

Both fail without this change (the limits check at exactly maxpos and maxneg for
widths 2, 5, 8 and 11) an... (continued)

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