Add downgrade GitHub Action workflow

[Krastanov-agent]

Summary

• Adds a downgrade workflow that tests the package with downgraded dependencies
• This ensures compatibility with older dependency versions
• Copied from QuantumSymbolics.jl following the same pattern

Test plan

• Verify the workflow runs on pull requests and pushes to master/main
• Confirm the downgrade action properly tests with Julia 1.10
• Check that the skip parameters are appropriate for this package

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 33.48%. Comparing base (cee7217) to head (bca2837).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #63      +/-   ##
==========================================
+ Coverage   33.25%   33.48%   +0.22%     
==========================================
  Files          15       15              
  Lines         442      442              
==========================================
+ Hits          147      148       +1     
+ Misses        295      294       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.