Function duration_struct_fields 

pub fn duration_struct_fields() -> Fields

The Arrow fields of a typed Cypher duration value (ADR 0009): signed Struct{months: Int64, days: Int64, seconds: Int64, nanos: Int64}. A struct (not Arrow Interval) because Parquet cannot persist Interval(MonthDayNano); the single source of truth shared by storage and the graphforge-rel query value. months/days/seconds are Int64 and seconds is split from nanos so billion-year duration.between/inSeconds spans fit (#920/#1011); nanos is nanoseconds-of-second, sharing the sign of seconds.