Function build_run_id 

pub fn build_run_id(start_ts_init: UnixNanos) -> RunId

Builds the <start_ts_init>-<short_uuid> run id used as the manifest key and on-disk file name.

The id is sortable by start time so directory listings produce chronological order; the short uuid suffix keeps it unique even when two kernels start at the same nanosecond on different machines.