Prevent CircularUKF state aliasing

[FlorianPfaff]

Bug

CircularUKF overrides AbstractFilter.filter_state for circular-Gaussian validation, but its setter stored the validated distribution object directly. The base AbstractFilter setter deep-copies assigned state, so the override broke the filter-state ownership contract: mutating the caller's Gaussian after assignment could silently mutate the filter estimate without another filter update.

Fix

After circular-specific validation, delegate storage to the established AbstractFilter.filter_state setter so assignment retains the base class's deep-copy semantics.

Regression coverage

Add a focused NumPy-backend regression that assigns a GaussianDistribution, mutates the original mean and covariance afterward, and verifies the CircularUKF estimate remains unchanged.

Validation

• branch is based on current main (f196ec41d1d8681e279936db9f1a0a3abfa276e7);
• final diff is limited to one production-line replacement plus a 30-line focused regression test;
• branch is 5 commits ahead and 0 behind because temporary branch-only setup commits cancel out of the final diff;
• Security checks and Dependency review pass on head 4f3ef2f1fc33c24e04c6e9469d40c50309da956d;
• CodeQL, MegaLinter, documentation examples, and release-notes checks are running, while the full test workflow is queued.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	26.65s
✅ JSON	prettier	7	0	0	0	1.47s
✅ JSON	v8r	7		0	0	4.05s
✅ MARKDOWN	markdownlint	68	0	0	0	1.92s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.54s
✅ PYTHON	black	1867	87	0	0	98.02s
✅ PYTHON	isort	1867	141	0	0	2.91s
✅ REPOSITORY	betterleaks	yes		no	no	2.15s
✅ REPOSITORY	checkov	yes		no	no	55.27s
✅ REPOSITORY	gitleaks	yes		no	no	17.7s
✅ REPOSITORY	git_diff	yes		no	no	0.32s
✅ REPOSITORY	secretlint	yes		no	no	79.36s
✅ REPOSITORY	syft	yes		no	no	6.12s
✅ REPOSITORY	trivy-sbom	yes		no	no	5.52s
✅ REPOSITORY	trufflehog	yes		no	no	32.61s
✅ YAML	prettier	11	0	0	0	0.75s
✅ YAML	v8r	11		0	0	11.9s
✅ YAML	yamllint	11		0	0	0.58s

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