fix: round negative fractions toward zero

[SashaMIT]

round(-1.25, 0) returned -2. math.Modf keeps the sign of the fractional part, so a negative fraction is never >= 0.5 and every negative value took math.Floor.

-1.25 is -1. -1.5 stays -2. -1.75 stays -2. The positive cases are unchanged. A custom threshold follows the same rule: -123.2329999 rounded to 2 digits with threshold 0.3 is -123.23, and -123.233 stays -123.24.

#488 changes the product to float64 so fused multiply-add does not move a positive fraction under the threshold. This patch leaves that line alone.

Red on Go 1.24.4: expected -1, actual -2. Green: TZ=UTC go test .