Function parse_mutation_path 

pub fn parse_mutation_path(absolute_path: &str) -> Result<AbsolutePath, Error>

Parses a raw string into the validated, normalized path a CommitRequest carries: absolute, normalized, and not the root.

This is the one named home for the invariant. Every surface that accepts a raw mutation path parses through it exactly once; planning re-asserts only the root guard ([ensure_mutation_path]) and never re-parses.