Function with_workload 

pub fn with_workload<F>(workload_path: &Path, mutate: F) -> Result<()>
where
    F: FnOnce(EditCtx<'_>) -> Result<String, String>,

Run a workload edit transaction. The closure produces the new source content; the driver handles lock, backup, roundtrip-parse, and atomic commit.

Errors:

• lock contention → kind WouldBlock with pid hint;
• I/O on read / write → propagated;
• mutation closure error → propagated, no on-disk change;
• post-mutate parse failure → backups rolled back, no on-disk change, error explains parse failure.