Fix empty checkpoints for custom JAX mappings

[MilesCranmerBot]

Summary

• omit non-picklable custom JAX/export mappings from checkpoints
• write checkpoints atomically so failed pickles do not leave empty files
• fall back to CSV backups when loading an empty/corrupt checkpoint

Fixes #1198

Tests

• uv run --python /usr/bin/python3.12 --with pytest --with 'jax[cpu]>=0.4,<0.6' --with 'juliacall>=0.9.28,<0.9.29' --with 'sympy>=1,<2' --with 'pandas>=0.21,<4' --with 'numpy>=1.13,<3' --with 'scikit-learn>=1,<2' --with 'click>=7,<9' --with 'typing-extensions>=4,<5' python -m pytest pysr/test/test_jax.py -k "checkpoint or empty"\n- python3 -m compileall -q pysr/sr.py pysr/test/test_jax.py

[codecov]

Codecov Report

❌ Patch coverage is 91.46341% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.34%. Comparing base (14cbe96) to head (aea7cde).

Files with missing lines	Patch %	Lines
pysr/sr.py	91.46%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1199      +/-   ##
==========================================
+ Coverage   94.24%   94.34%   +0.09%     
==========================================
  Files          21       21              
  Lines        1686     1714      +28     
==========================================
+ Hits         1589     1617      +28     
  Misses         97       97              

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[MilesCranmer]

@MilesCranmerBot it feels like we are loading up this PySRRegressor with too much stuff In particular there are a bunch of new functions here which are not really "core" operations to the regressor. I believe there is a nice way to refactor this. Please do so.

[MilesCranmerBot]

Refactored in 2e59bfd: moved the checkpoint/pickle/export-column helper logic into pysr/_checkpoint.py, leaving PySRRegressor with thin call sites.

Checks run:

• uvx pycln pysr/sr.py pysr/_checkpoint.py pysr/test/test_jax.py
• uvx black --check pysr/sr.py pysr/_checkpoint.py pysr/test/test_jax.py
• uvx isort --check-only pysr/sr.py pysr/_checkpoint.py pysr/test/test_jax.py
• uv run --python /usr/bin/python3.12 --with pytest --with "jax[cpu]>=0.4,<0.6" python -m pytest pysr/test/test_jax.py -k "checkpoint_custom_jax_mapping or empty_checkpoint"