`skyscraper/fp-rounding/src/lib`: Distinguish from Negative

[recmo]

On 2025-08-27 in d91b2f2 “Merge pull request #142 from worldfnd/sl/skyscraper-refactor”:

Distinguish from Negative

            assert_ne!(a - b, a);
        }
        RoundingDirection::Zero => {
            assert_eq!(a + b, a);
            assert_ne!(a - b, a);
            // TODO: Distinguish from Negative
        }
    }
}

#[cfg(test)]

From skyscraper/fp-rounding/src/lib.rs:102