Integer and long GPU reductions

GPU offload supports proven integer and long reductions whose method descriptor returns )I or )J. The analyzer requires a counted loop, a loop-carried accumulator that feeds the returned local, array reads, and no array store. The interpreter receives the downloaded scalar through DispatchOutcome::HandledWithValue in vm/src/runtime/offload.rs.

The PTX epilogue uses the one-operand reduction form red.global.add[.u64] [ptr], value;. This matters: the older two-operand atom.global.add [ptr], value; form was rejected by ptxas, silently forcing CPU fallback. ptxas_round_trip_dot_reduction guards the emitted form.

Float and double reductions intentionally remain on CPU. Their atomic-add order is not bit-identical to Java's sequential floating-point accumulation.

See also the historical validation record.