dtypeOfSchema

Derives the dtype an ArrowSchema describes. The flat tier maps through dtypeOfArrowFormat; the rich tier parses the tree: +L/+l with one flat element child becomes LIST (the 32-bit +l offsets widen on import; nesting rejects), a dictionary over int32 codes with utf8/large_utf8 labels becomes CATEGORICAL, and STRUCT rejects (the storage tier drops field names).