supertypeOf

The common dtype both operands of an arithmetic operation are cast to before the kernel runs (D12, ADR-2026-10-18-05; one dispatch on one kind, fourteen instantiations per operation, not 196).

Rules (pinned by the matrix unittest and printed in doc/DDN-DF.md): same kind stays; BOOLEAN with a numeric kind gives the numeric kind; two signed or two unsigned integers give the wider; signed with unsigned gives the signed kind when it is strictly wider, otherwise the next wider signed kind; UINT64 with any signed kind gives FLOAT64; FLOAT32 with the 8- and 16-bit integers stays FLOAT32, with the 32- and 64-bit integers becomes FLOAT64; FLOAT64 with anything numeric is FLOAT64. STRING and the temporal kinds have no arithmetic supertype in this numeric lattice. Temporal arithmetic is not banned: it is handled by the dedicated typed DATE/DATETIME/TIME/DURATION rules instead of this function. Comparisons do not use this function either: they require the same kind (temporal with temporal, string with string) or the numeric supertype.