Fix JAX UKF sigma-point updates

[FlorianPfaff]

Summary

• remove the explicit JAX rejection from Julier/Merwe sigma-point constructors
• build sigma-point weights and matrices functionally instead of via in-place array writes
• build UKF propagated/measurement sigma matrices via stack, so immutable backends such as JAX do not fail at row assignment
• allow the Euclidean UKF public methods on JAX while keeping the existing PyTorch guard
• add a backend-portable JAX subprocess regression test for sigma-point generation and a predict/update cycle

Rationale

The JAX backend uses immutable arrays. The existing sigma-point and UKF helper implementations used NumPy-style in-place row assignment, and the public Euclidean UKF additionally rejected JAX before reaching the helper. This made an otherwise backend-expressible UKF path unusable on JAX.

Validation

• Branch was recreated from current main before opening this PR.
• Could not run local pytest in the execution container because direct GitHub checkout/network access is unavailable here; CI should exercise the new backend_portable JAX regression and the existing suite.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	43.03s
✅ JSON	prettier	7	0	0	0	1.11s
✅ JSON	v8r	7		0	0	4.9s
✅ MARKDOWN	markdownlint	66	0	0	0	1.65s
✅ MARKDOWN	markdown-table-formatter	66	0	0	0	0.74s
✅ PYTHON	black	767	1	0	0	37.51s
✅ PYTHON	isort	767	1	0	0	2.69s
✅ REPOSITORY	checkov	yes		no	no	43.71s
✅ REPOSITORY	gitleaks	yes		no	no	13.73s
✅ REPOSITORY	git_diff	yes		no	no	0.1s
✅ REPOSITORY	secretlint	yes		no	no	26.61s
✅ REPOSITORY	syft	yes		no	no	5.64s
✅ REPOSITORY	trivy-sbom	yes		no	no	4.37s
✅ REPOSITORY	trufflehog	yes		no	no	20.65s
✅ YAML	prettier	11	0	0	0	0.59s
✅ YAML	v8r	11		0	0	10.03s
✅ YAML	yamllint	11		0	0	0.48s

Notices

📣 MegaLinter 9.5.0 is out! Discover the new features and security recommendations in the release announcement. (Skip this info by defining SECURITY_SUGGESTIONS: false)

See detailed reports in MegaLinter artifacts

Your project could benefit from a custom flavor, which would allow you to run only the linters you need, and thus improve runtime performances. (Skip this info by defining FLAVOR_SUGGESTIONS: false)

• Documentation: Custom Flavors
• Command: npx mega-linter-runner@9.5.0 --custom-flavor-setup --custom-flavor-linters PYTHON_BLACK,PYTHON_ISORT,COPYPASTE_JSCPD,JSON_V8R,JSON_PRETTIER,MARKDOWN_MARKDOWNLINT,MARKDOWN_MARKDOWN_TABLE_FORMATTER,REPOSITORY_CHECKOV,REPOSITORY_GIT_DIFF,REPOSITORY_GITLEAKS,REPOSITORY_SECRETLINT,REPOSITORY_SYFT,REPOSITORY_TRIVY_SBOM,REPOSITORY_TRUFFLEHOG,YAML_PRETTIER,YAML_YAMLLINT,YAML_V8R

Show us your support by starring ⭐ the repository