Fix MEM-EKF pseudo-measurement covariance

[FlorianPfaff]

Summary

• replace the MEM-EKF quadratic pseudo-measurement raw fourth-moment matrix with the centered Gaussian covariance for [dx², dx dy, dy²]
• keep the existing pseudo-mean subtraction and route the update through the centered covariance helper
• add regression tests for correlated and diagonal innovation covariances

Testing

• Not run locally; the execution sandbox could not resolve github.com for a local checkout. The regression tests are included for CI.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	55.86s
✅ JSON	prettier	7	0	0	0	0.92s
✅ JSON	v8r	7		0	0	4.5s
✅ MARKDOWN	markdownlint	68	0	0	0	1.82s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.77s
✅ PYTHON	black	879	0	0	0	44.42s
✅ PYTHON	isort	879	0	0	0	2.55s
✅ REPOSITORY	checkov	yes		no	no	42.71s
✅ REPOSITORY	gitleaks	yes		no	no	10.09s
✅ REPOSITORY	git_diff	yes		no	no	0.15s
✅ REPOSITORY	secretlint	yes		no	no	26.95s
✅ REPOSITORY	syft	yes		no	no	2.98s
✅ REPOSITORY	trivy-sbom	yes		no	no	7.93s
✅ REPOSITORY	trufflehog	yes		no	no	19.07s
✅ YAML	prettier	11	0	0	0	0.72s
✅ YAML	v8r	11		0	0	10.9s
✅ YAML	yamllint	11		0	0	0.46s

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