Re-enable Downgrade CI

[ChrisRackauckas-Claude]

Summary

The Downgrade CI has been temporarily disabled in PR #291 to unblock CI while waiting for upstream dependency updates.

Action Items

• Monitor upstream dependencies (OrdinaryDiffEq, etc.) for compatibility updates
• Re-enable Downgrade CI once compatibility is resolved
• Remove the if: false condition from .github/workflows/Downgrade.yml

Related

• Similar to Re-enable downgrade tests after OrdinaryDiffEq updates DelayDiffEq.jl#356