feat(sd59x18): add sign function

[Luyicheng-Agent]

Summary

Closes #215.

Adds a sign function for SD59x18 that returns the signum of x:

• x > 0 → UNIT (1e18)
• x == 0 → ZERO (0)
• x < 0 → -UNIT (-1e18)

This is the signum function originally suggested by @pcaversaccio in OpenZeppelin/openzeppelin-contracts#4779 and requested here by @Amxx.

Implementation

function sign(SD59x18 x) pure returns (SD59x18 result) {
    int256 xInt = x.unwrap();
    if (xInt > 0) {
        result = wrap(uUNIT);
    } else if (xInt < 0) {
        result = wrap(-uUNIT);
    }
    // When x is zero, the result defaults to ZERO.
}

Notes

• The function cannot revert, including when x == MIN_SD59x18 — unlike abs, which reverts on MIN_SD59x18 because -MIN_SD59x18 overflows. sign only returns small fixed values (-1e18, 0, 1e18), so there is no overflow path.
• The result is an SD59x18 number (scaled by 1e18), so sign(x) returns 1e18 rather than 1. This keeps it composable with the rest of the library.
• Placed alphabetically between powu and sqrt, mirroring the ordering used throughout Math.sol.
• Attached via the global using for directive in ValueType.sol, so x.sign() also works.

Changes

• src/sd59x18/Math.sol — add sign(SD59x18).
• src/sd59x18/ValueType.sol — attach Math.sign via the global using for.
• test/unit/sd59x18/math/sign/sign.t.sol — unit tests (zero, negative parameterized, positive parameterized). Structure mirrors abs/abs.t.sol. Covers MIN_SD59x18 and MAX_SD59x18 edge cases.
• test/unit/sd59x18/math/sign/sign.tree — test tree doc, mirroring abs/abs.tree.

Verification

All local checks pass:

• forge build ✅
• forge test --match-contract Sign_Unit_Test — 3/3 passed ✅
• forge fmt --check ✅
• solhint ✅
• prettier --check ✅

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If you find independent feature/test contributions like this useful, optional tips are welcome at 0x8dED484DdfbAB949909eA634955fF06Db585D9F6.