Function is_na_real 

pub(crate) fn is_na_real(value: f64) -> bool

Check if an f64 value is R’s NA_real_ (a specific NaN bit pattern).

This is different from f64::is_nan() which returns true for ALL NaN values. R’s NA_real_ is a specific NaN with a particular bit pattern, while regular NaN values (e.g., from 0.0/0.0) should be preserved as valid values.