Avoid overflow when symmetrizing ellipse matrices

[FlorianPfaff]

Summary

• scale each operand before averaging an ellipse matrix with its transpose
• avoid intermediate overflow for valid finite matrices near the floating-point limit
• add a backend-portable strict-overflow regression test

Bug

0.5 * (matrix + matrix.T) can overflow while forming the intermediate sum even when the mathematically averaged result is finite. This affects the public pyrecest.tracking.symmetrize helper and all ellipse covariance/extent paths that use it.

Fix

Compute 0.5 * matrix + 0.5 * matrix.T instead. The result is algebraically equivalent, backend-friendly, and does not double finite entries before scaling.

Validation

The regression uses np.finfo(np.float32).max entries and runs under strict NumPy overflow handling. The previous implementation produces an overflow before scaling; the revised implementation preserves the finite matrix exactly across the supported backends.

[github-actions]

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[FlorianPfaff]

Closing as superseded by #5239. Both PRs make the identical production change in tracking/ellipse_geometry.py; #5239 is the newer replacement and additionally exercises integer-dtype overflow through the public covariance projection path.