FixedDecimal._quotient

Quotient of two runtime-form operands at an explicit result scale, rounded half-even (the single rounding rule, D3/D4).

Algorithm: integer part and fraction digits come from 256-bit long division of |c1| × 10^(s2−s1) (or the dual form when s1 > s2) by |c2|; the digit past the result scale decides half-even rounding (remainder tracked exactly). Zero divisor throws; a rounded coefficient beyond the precision throws.