public Map<ShapeId, Pair<Trait, Trait>> getTraitDifferences() {
        return traitDiff;
    }
Gets the trait differences between the old and new shape.

<p>The returned map is a mapping of a trait name to a pair in which the
left side of the pair contains the nullable old trait value, and the
right side of the pair contains the nullable new trait value. The left
side will be null if the trait was added, the right side will be null
if the trait was removed, and both traits will be present if the trait
changed.

@return Returns a map of each changed trait name to a pair of the old and new trait values.
smithy-diff/src/main/java/software/amazon/smithy/diff/ChangedShape.java