Fix grad symmetry and release metadata gaps

[t-uda]

Summary

• return symmetric covariance gradients from coef_from_cov_grad and update the finite-difference regression to validate grad_cov directly
• export the C++ build metadata symbols with ELLPHI_EXPORT so Windows wheels expose build_info metadata reliably
• build Linux release wheels for Python 3.10, 3.11, and 3.12 to match supported interpreters

Testing

• env MPLCONFIGDIR=/tmp/mpl poetry run pytest tests/test_coef_from_cov_grad.py tests/test_cpp_backend.py tests/test_cli.py
• env MPLCONFIGDIR=/tmp/mpl poetry run python scripts/build_tangency_cpp.py
• env MPLCONFIGDIR=/tmp/mpl poetry run pytest tests/test_coef_from_cov_grad.py tests/test_cpp_backend.py tests/test_cli.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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