math: handle extreme orders in Jn and Yn - #81573
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Use unsigned order magnitudes so MinInt can be represented, and convert recurrence coefficients to float64 before doubling them. Stop the backward recurrence when its scaling factor underflows, keeping the sign implied by the order and argument. This avoids an impractically long recurrence for inputs such as Jn(MaxInt, 1). Add tests for extreme orders, signs, special values, subnormal results, and large arguments where the result must not be forced to zero. Fixes golang#81489
Rename nn to absN in Jn and Yn to make the unsigned absolute order explicit. The type, arithmetic, control flow and tests are unchanged. Updates golang#81489
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Use unsigned order magnitudes so MinInt can be represented, and convert
recurrence coefficients to float64 before doubling them.
Stop the backward recurrence when its scaling factor underflows, keeping
the sign implied by the order and argument. This avoids an impractically
long recurrence for inputs such as Jn(MaxInt, 1).
Add tests for extreme orders, signs, special values, subnormal results,
and large arguments where the result must not be forced to zero.
The existing recurrence is retained; enormous orders near the argument
can still require time proportional to the order.
The reported cases fail or time out before the fix and pass afterward.
Tests pass for math/..., with race and linux/386 checks for math.
A differential probe preserves 15,352 sampled outputs, and a separate
rational-series calculation checks the subnormal reference values.
The complete Go repository test suite was not run for this change.
Fixes #81489