Function write_record 

pub fn write_record(root: &Path, record: &GateRecord) -> Result<PathBuf>

2026-09-26: Write one gate record and return its path. Creates the parent directory; commits nothing. The path is record_path_for unless that file holds a FAIL, in which case it is the re-run name. Errors when the re-run name holds a FAIL too, or when either file exists but is not a readable record.