Initialize hemisphere particle filters on the manifold

[FlorianPfaff]

What changed

• Replace empty(...) particle locations in HyperhemisphericalParticleFilter with deterministic north-pole unit vectors.
• Apply the same fix component-wise in HyperhemisphereCartProdParticleFilter.
• Add regressions that verify the constructors produce the expected valid hemisphere directions.

Why

The constructors previously passed uninitialized memory directly to Dirac distributions. Dirac weight validation does not normalize particle locations, while downstream hemisphere moments and mean-axis calculations assume valid directional samples. Depending on backend and allocator state, the initial filter could therefore start from zero, arbitrary, or off-manifold particle locations.

Validation

• Added constructor-level geometry regressions for both affected filters.
• Changes are limited to initialization and its tests; later state-setting and prediction behavior is unchanged.

Created as a draft so repository CI can validate all supported backends before merge.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Max errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no		no	8.39s
✅ JSON	prettier	7	0	0		0	0.72s
✅ JSON	v8r	7		0		0	4.53s
✅ MARKDOWN	markdownlint	68	0	0		0	2.08s
✅ MARKDOWN	markdown-table-formatter	68	0	0		0	0.41s
✅ PYTHON	black	1983	11	0		0	67.55s
✅ PYTHON	isort	1983	21	0		0	3.01s
✅ REPOSITORY	betterleaks	yes		no		no	1.35s
✅ REPOSITORY	checkov	yes		no		no	37.5s
✅ REPOSITORY	git_diff	yes		no		no	0.13s
✅ REPOSITORY	secretlint	yes		no		no	108.45s
✅ REPOSITORY	syft	yes		no		no	3.8s
✅ REPOSITORY	trivy-sbom	yes		no		no	2.46s
✅ YAML	prettier	11	0	0		0	0.85s
✅ YAML	v8r	11		0		0	17.86s
✅ YAML	yamllint	11		0		0	1.19s

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