Re-enable downgrade tests

[ChrisRackauckas-Claude]

Summary

The downgrade tests have been temporarily disabled in #207 to unblock CI.

Context

Related to #206 - the fix for the StackOverflowError with Double64 types adds DoubleFloats as a test dependency, which may affect downgrade compatibility.

Action items

• Investigate downgrade test failures
• Re-enable downgrade tests once compatibility is resolved

To re-enable

Remove the if: false line from .github/workflows/Downgrade.yml.