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fix(dfloat): store the leftover DPD trailing digits (#1486)

A DPD dfloat keeps its leading digit in the combination field and the other
ndigits - 1 in 10-bit declets of three. When ndigits - 1 is not a multiple of 3
(ndigits % 3 != 1), one or two digits are left over, and dpd_trailing_bits()
reserves the top 4 or 7 trailing bits for them. But dpd_encode_trailing_wide() and
dpd_decode_trailing_wide() looped over full declets only, so the leftover digits were
never written or read. dfloat<6,6,DPD>(123456) read back as 100456, and
dfloat<8,6,DPD>(12345678) as 10345678. decimal32, decimal64 and decimal128 (7, 16
and 34 digits) have no leftover digits and are unchanged.

Both functions now write and read the leftover group at the top of the trailing
field: a lone digit in 4 bits (DPD and BCD coincide there), a pair as the 7-bit DPD
group of #1480 (dpd_encode_2digits / dpd_decode_2digits).
dpd_encode_significand() / dpd_decode_significand() in dpd_codec.hpp, which only
the codec test calls, did handle the leftover pair, but packed it as
(d1 << 4) | d0: 8 bits in a 7-bit field, so 200,000 of the 1,000,000 six-digit
significands failed to round-trip. They now use the same helpers.

New test static/float/dfloat/conversion/dpd_trailing.cpp (dfloat_dpd_trailing):
- The same random decimals, with 8s and 9s forced into the leftover positions, must
  unpack to identical sign, exponent and significand in BID and DPD, with BID holding
  exactly the digits written. + - * / on pairs must agree, at every precision from 2
  to 12 digits and at decimal64 (13 to 19 digits and decimal128 at higher levels).
- The significand codec must round-trip exhaustively at 5 and 6 digits (7 and 8 at
  higher levels).

The operand pairs stay within two decades of each other, and their significands end
in a nonzero digit. Wider gaps and short divisors overflow the significand in both
encodings (#1484, filed from this work, as is the parse bug #1485). On main the test
fails at exactly th... (continued)

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