[chiliec]

What

Context.Pow with an exact negative integer exponent no longer preserves the exponent that the General Decimal Arithmetic power operation specifies.

At precision 34, Pow(2, -1) returns 0.5000000000000000000000000000000000 (exponent -34) and reports Rounded; Pow(10, -2) returns 0.0100...0. apd v3.0.0 returned 0.5 and 0.01 with no flags. The GDA power operation says an exact integer-power result carries the exponent it would have under repeated multiplication (with the reciprocal of the base for a negative exponent), e.g. power('2','-3') ==> '0.125' (http://speleotrove.com/decimal/daops.html#refpower). Python's decimal returns 0.5 at the same precision.

The regression came in with 04d1483 ("apd: don't strip trailing zeros in Quo and Sqrt", v3.1.0): integerPower divides one by the computed power with Quo, which now keeps precision digits, so Pow inherited a division convention the power spec does not share. The GDA suite in gda_test.go compares with Cmp, which ignores the exponent, so powx101 power 2 -1 -> 0.5 still passed.

Reported in #161 (found via cue-lang/cue, which uses apd for arbitrary-precision numbers). This PR addresses the exact-exponent part only, not the separate precision-0 question in that issue.

Fix

In integerPower, after computing the reciprocal for a negative exponent, Reduce the quotient when it is exact (!Inexact). An exact reciprocal of an integer power has no genuine trailing zeros, so this strips only the precision padding and restores the spec exponent; the Rounded flag is no longer raised for exact results. Inexact reciprocals (e.g. 3**-1) are left untouched.

Tests

Added TestPowExactInteger in decimal_test.go asserting the text, exponent, and condition flags for exact positive and negative integer powers plus one inexact case.

Validation (go1.23.2, linux/amd64):

$ go test -run TestPowExactInteger -v ./
--- PASS: TestPowExactInteger (0.00s)
    --- PASS: TestPowExactInteger/2**-1
    --- PASS: TestPowExactInteger/2**-3
    --- PASS: TestPowExactInteger/10**-2
    --- PASS: TestPowExactInteger/5**-2
    --- PASS: TestPowExactInteger/2**3
    --- PASS: TestPowExactInteger/10**3
    --- PASS: TestPowExactInteger/3**-1
PASS

$ go test ./
ok  	github.com/cockroachdb/apd/v3	3.372s

The full suite (including the GDA gda_test.go power tests) passes on the branch.