stillwater-sc / universal / 34789736838

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fix(dfloat): round to nearest, ties to even, everywhere (#1490)

numeric_limits<dfloat>::round_style declares round_to_nearest, and IEEE 754-2008
decimal arithmetic defaults to roundTiesToEven, but dfloat mixed three policies:
+ - * / truncated (decimal32 2/3 = 0.6666666), conversion from double rounded half
up (0.6666667), parse and str(precision) truncated, and the far-gap shortcut in +/-
returned the nearest value. Every path now rounds to nearest, ties to even.

- round_to_ndigits() rounds a significand of either width to ndigits from the digits
  it drops plus a sticky flag (a nonzero amount below the last digit), carrying
  99...9 into a new digit. normalize_and_pack() and normalize_wide_and_pack() call
  it and check for overflow after rounding, so a carry past maxpos gives inf.
- + and - round the exact double-width sum. The shortcut returns the larger operand
  only when the smaller one's leading digit is more than ndigits + 1 below it, where
  the nearest value is the larger operand with either sign (the smaller one is under
  a tenth of a unit in the last place). Closer operands take the exact sum.
- * rounds the exact double-width product; it used to trim by truncation.
- / carries an inexact quotient to ndigits + 1 digits, a guard digit, and passes the
  remainder as sticky.
- Parse keeps one guard digit past the precision, and any nonzero digit after it
  sets sticky.
- Conversion from double rounds its scaled value half to even instead of half up.
- str(precision), and so operator<< under std::setprecision, round the printed digits
  to nearest with ties to even, carrying 9s. std::scientific with
  std::setprecision(3) printed 2/3 as 6.66e-1; it now prints 6.67e-1.
- Conversion from integers rounds through normalize_and_pack. decimal32 from
  123456789 is 1.234568e8, where it was 1.234567e8.

Tests move to the new policy:
- arithmetic/oracle.cpp: every result must be the exact value rounded half-even on
  digit strings. Division runs to... (continued)

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