pub fn save(store: &ArtifactStore, record: &mut RunRecord) -> Result<PathBuf>Expand description
2026-09-26: Write record, setting its run_id to the file stem chosen,
and return the path.
The name is run-<unix_nanos>, zero-padded to 19 digits, so a filename
sort is a time sort while the nanos have 19 digits (until 2286). A taken
name moves the nanos up by one until a name is free.