BUG: fix divide by zero in quartic_restricted when y is constant

[Omesh37]

Description

pvanalytics/util/_fit.py raised RuntimeWarning: divide by zero encountered in scalar divide when y contains constant values.

In quartic_restricted, the R² denominator np.sum((y - np.mean(y))**2) evaluates to zero when all values in y are identical. This caused a RuntimeWarning in:

• test_orientation.py::test_ghi_not_tracking
• test_orientation.py::test_power_tracking_perturbed

Fix: Check if ss_tot == 0 before dividing. If so, return 0.0 since R² is undefined for constant data (no variation to explain).

• Closes BUG: divide by zero RuntimeWarning in _fit.quartic_restricted when y is constant #234
• Pull request is nearly complete and ready for detailed review.
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