Function parse_open_fraction_or_default 

pub fn parse_open_fraction_or_default(raw: Option<String>, default: f64) -> f64

Parse a fraction strictly inside the open interval (0, 1). Used for parameters where 0.0 or 1.0 produce algorithmic degeneracy (e.g. PPR_ALPHA=1.0 makes restart probability zero, yielding all-zero rankings).