fix: check y == 0 before abs(x) in SD59x18 powu to handle MIN_SD59x18

[BhariGowda]

Summary

powu(MIN_SD59x18, 0) currently reverts instead of returning UNIT.

abs() is called before the y == 0 short-circuit check, and abs(MIN_SD59x18)
always reverts. Mathematically, x^0 = 1 for all x including MIN_SD59x18.

Compare with pow() which correctly handles y == 0 before touching x.

Fix

Move the y == 0 guard before the abs(x) call:

function powu(SD59x18 x, uint256 y) pure returns (SD59x18 result) {
    if (y == 0) return UNIT;
    uint256 xAbs = uint256(abs(x).unwrap());

[PaulRBerg]

Good catch. Thanks @BhariGowda. I've fixed the NatSpec and the formatting, and updated the tests. Merging.

[PaulRBerg]

The fix is available in v4.1.2, already shipped to npm.