Stabilize linear-update covariance symmetrization

[FlorianPfaff]

Bug

linear_update_planning._symmetrized(...) averaged covariance-like matrices as

0.5 * (matrix + matrix.T)

For large but finite values, the intermediate addition can overflow even though the mathematically correct symmetric average is finite. A concrete one-dimensional covariance equal to 0.75 * np.finfo(float).max therefore raises FloatingPointError under strict NumPy overflow handling before the planner can perform an otherwise valid update.

This helper is used both while validating component covariance matrices and when symmetrizing nominal/effective innovation covariance matrices.

Fix

Scale each operand before adding it:

0.5 * matrix + 0.5 * matrix.T

The expression is algebraically equivalent for ordinary finite inputs but avoids doubling a large entry before the factor of one half is applied.

Regression coverage

Add a public-path regression through plan_linear_measurement_update(...) with a covariance of 0.75 * np.finfo(float).max under np.errstate(over="raise", invalid="raise"). The test verifies that the plan remains accepted and that the nominal innovation covariance stays finite and exactly retains the large finite value.

Validation

• independently reproduced the pre-fix FloatingPointError: overflow encountered in add;
• verified the scale-first symmetric average remains finite and equals the original covariance;
• final PR diff is limited to one production-line replacement and a focused 17-line regression test;
• branch is based on current main (f196ec41d1d8681e279936db9f1a0a3abfa276e7);
• GitHub Actions is authoritative for the full test, lint, security, documentation, and integration matrix.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	15.81s
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✅ JSON	v8r	7		0	0	3.12s
✅ MARKDOWN	markdownlint	68	0	0	0	1.07s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.46s
✅ PYTHON	black	1866	87	0	0	56.71s
✅ PYTHON	isort	1866	140	0	0	1.91s
✅ REPOSITORY	betterleaks	yes		no	no	1.96s
✅ REPOSITORY	checkov	yes		no	no	34.92s
✅ REPOSITORY	gitleaks	yes		no	no	10.76s
✅ REPOSITORY	git_diff	yes		no	no	0.16s
✅ REPOSITORY	secretlint	yes		no	no	47.69s
✅ REPOSITORY	syft	yes		no	no	3.65s
✅ REPOSITORY	trivy-sbom	yes		no	no	4.42s
✅ REPOSITORY	trufflehog	yes		no	no	20.63s
✅ YAML	prettier	11	0	0	0	0.44s
✅ YAML	v8r	11		0	0	7.94s
✅ YAML	yamllint	11		0	0	0.44s

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