Jacobian bugfix

[cmdupuis3]

Closes #1645

Overview

This PR resolves incorrect Jacobian calculations, described in #1645.

The fix is fairly simple, we basically just rename the node jacobian results so they don't clobber the accumulators. I also added a regression test.

Bonus: The Grid.face_jacobian property was technically unreachable due to a logic error. One-line fix.

PR Checklist

General

• An issue is created and linked
• Added appropriate labels (if your uxarray repo permissions allow it)
• Filled out Overview and Expected Usage (if applicable) sections

Testing & Benchmarking

• Tests are not too basic (such as simply calling a function and nothing else)
• Tests cover all major paths in your new functions

[cmdupuis3]

pre-commit.ci autofix

[Sevans711]

Fixes look clean, fully covers the scope of the original issue, and I confirmed the regression tests fail on main but succeed on this branch. Looks ready to merge!

My only tiny suggestion (non-blocking) would be to maybe add something like "regression test for issue # 1645" to the test docstring for easy/quick future reference.

[rajeeja]

Verified locally: the accumulator no longer gets clobbered, face areas are unchanged, and the new regression tests plus the rest of test_areas.py pass. The face_jacobian property fix is correct too.