flagTransaction

Flags id (a captured InspectorTransactionUi) into the evidence tray. The subject is materialized fresh from whatever this data source's own capture store currently holds for id -- never re-derived from a value the caller already has cached -- so the stored snapshot matches exactly what the dashboard's own flag route would have produced for the same transaction. InspectorCommandResult.Invalid covers both "already flagged" and "evidence tray full"; InspectorCommandResult.Unavailable covers no durable store being wired up.