Add General Decimal Arithmetic ordering, adjacency, and exponent operations

[Btocode]

Adds the following General Decimal Arithmetic (GDA) / IEEE 754 decimal operations that apd was missing:

• Context.Max, Min, MaxAbs, MinAbs — numeric and by-magnitude minimum and maximum, with the specification's NaN and total-order tie-break rules.
• Context.NextPlus, NextMinus, NextToward — the representable value adjacent to an operand in a given direction.
• Context.LogB, ScaleB — the adjusted exponent, and scaling by a power of ten.
• Context.CopySign — the magnitude of one operand with the sign of another.
• Decimal.SameQuantum and Decimal.CmpTotalMag — companions to the existing CmpTotal.

The operations follow the GDA specification, including the condition flags they raise (InvalidOperation, DivisionByZero, Overflow, Underflow, Subnormal, Inexact, Rounded, Clamped). Implementation notes worth a reviewer's attention:

• Max/Min treat a quiet NaN operand as absent (return the other operand); only a signaling NaN raises InvalidOperation. Ties (numerically equal operands) are broken by total order, so e.g. Max(1.0, 1.00) returns 1.0.
• The next-* operations compute increments on non-negative magnitudes (via nextPlus(x) == -nextMinus(-x)) to avoid relying on directed rounding of negative subnormals, and clamp directed-rounding overflow to Nmax rather than infinity.

New table-driven tests cover signed zeros, subnormals near Etiny, values near Nmax, infinities, and quiet/signaling NaNs. The expected values were additionally cross-checked against the Python standard library's decimal module (which implements the same specification) over tens of thousands of randomized inputs.