Function reify_variable_value_range 

fn reify_variable_value_range(
    css: &str,
    references: &[SubstitutionFunctionReference],
    reference_index: &mut usize,
    start: usize,
    end: usize,
) -> Result<UnparsedValue, ()>

Reify a slice of the CSS string into UnparsedSegment entries.

References are stored in source order, with outer substitution functions inserted before references in their fallback. The shared reference_index relies on this ordering to recurse into fallbacks without reprocessing nested referecences.