Function round_decimal_f64 

pub(crate) fn round_decimal_f64(v: f64) -> f64

Rounds an f64 to 14 decimal places using pure arithmetic.

Eliminates the string formatting roundtrip entirely. For values like 4.6 that can’t be exactly represented in IEEE 754, this cleans up trailing noise (e.g. 4.6000000000000005 → 4.6).

Splits into integer and fractional parts before scaling to avoid overflow: large SAS datetime values (~1.9e9) multiplied by 1e14 would exceed f64’s exact integer range (2^53), causing rounding errors.