Function write_with_lock 

pub fn write_with_lock(
    path: &Path,
    marker: &str,
    content_fn: impl FnOnce() -> String,
    append_fn: Option<impl FnOnce(&str) -> String>,
) -> Result<(), ExportError>

Write content to a file, with idempotent marker checking.

If the file already contains a marker string, the write is skipped. Thread-safe via per-file locking.