fix: fail fast on checkpoint version mismatch

[MilesCranmerBot]

Fixes #941.

Adds checkpoint metadata (PySR + SymbolicRegression backend + Julia version) and validates it before unpickling, so version mismatches raise a clear actionable error instead of opaque deserialization failures.

TDD:

• Adds unit test for mismatch raising a clear error.
• Adds unit test that checkpoint write includes metadata.

Local test:

• python -m pytest pysr/test/test_main.py -k checkpoint_metadata

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.37%. Comparing base (2ac7e71) to head (53f20f0).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
pysr/sr.py	94.11%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1135      +/-   ##
==========================================
+ Coverage   93.91%   94.37%   +0.45%     
==========================================
  Files          21       21              
  Lines        1643     1723      +80     
==========================================
+ Hits         1543     1626      +83     
+ Misses        100       97       -3     

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