Function batch_mutate 

pub fn batch_mutate(
    project_root: &ProjectRoot,
    mutations: Vec<MutationRequest>,
    file_locks: Option<&FileLockManager>,
) -> Result<BatchMutationResult, AqlError>

Execute multiple mutations in parallel across files.

Groups mutations by file, sorts within each file by byte offset (descending) to avoid offset shifting, parallelizes across files via rayon, and writes each file once after all its mutations are applied.