feat(sd59x18): add sign function (#285)

* feat(sd59x18): add sign function

Closes #215.

Adds a sign (signum) function for SD59x18 that returns UNIT (1e18) for positive inputs, ZERO (0) for zero, and -UNIT (-1e18) for negative inputs. Cannot revert, including for MIN_SD59x18. Requested by @pcaversaccio, filed by @Amxx. Tests mirror abs.t.sol.

* refactor(sd59x18): polish sign implementation

- make all sign outcomes explicit\n- avoid compiler warnings from name shadowing and test mutability

---------

Co-authored-by: Luyicheng-Agent <agent@luyicheng.local>
Co-authored-by: Paul Berg <prberg@proton.me>