Function backfill_session_ids 

pub fn backfill_session_ids(reg: &Registry, shared_dir: &Path) -> usize

Populate session_id for registry records that lack one, from the agent’s largest provider session. Idempotent (skips records that already carry a non-empty id). Returns the number populated. Best-effort per record — a record with no source_agents_base or no transcript is skipped, never fatal.