Function LinePosToOffset 

fn LinePosToOffset(
    LineOffsets: &[usize],
    Source: &str,
    Line: usize,
    Character: usize,
) -> usize

Resolve (line, character) to an absolute byte offset. Character is counted in UTF-16 code units to match VS Code’s Range/Position semantics. Falls back gracefully when line/char is past EOF.