Fix hyperhemispherical uniform sampling

[FlorianPfaff]

Summary

• Fix HyperhemisphericalUniformDistribution.sample to mirror each sampled point based on its own last coordinate.
• Update the sampler comment to describe the hemisphere projection accurately.
• Add regression tests for per-row mirroring and the single-sample equator case.

Root cause

HypersphericalUniformDistribution.sample(n) returns samples shaped (n, dim + 1), but the hyperhemisphere sampler checked s[-1, :], which reads the last sampled row instead of the last coordinate of every sample. For n == 1, that could force all coordinates positive instead of only projecting to the upper hemisphere.

Validation

• $env:PYTHONPATH='src'; $env:PYRECEST_BACKEND='numpy'; python -m pytest -q tests/distributions/test_hyperhemispherical_uniform_distribution.py
• $env:PYTHONPATH='src'; $env:PYRECEST_BACKEND='numpy'; python -m pytest -q tests/distributions/test_hyperhemispherical_uniform_distribution.py tests/distributions/test_hyperspherical_uniform_distribution.py
• $env:PYTHONPATH='src'; $env:PYRECEST_BACKEND='pytorch'; python -m pytest -q tests/distributions/test_hyperhemispherical_uniform_distribution.py
• $env:PYTHONPATH='src'; $env:PYRECEST_BACKEND='jax'; python -m pytest -q tests/distributions/test_hyperhemispherical_uniform_distribution.py
• python -m black --check src/pyrecest/distributions/hypersphere_subset/hyperhemispherical_uniform_distribution.py tests/distributions/test_hyperhemispherical_uniform_distribution.py
• python -m ruff check src/pyrecest/distributions/hypersphere_subset/hyperhemispherical_uniform_distribution.py tests/distributions/test_hyperhemispherical_uniform_distribution.py

[github-actions]

✅MegaLinter analysis: Success

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✅ PYTHON	mypy	429		0	0	5.21s
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✅ REPOSITORY	gitleaks	yes		no	no	7.71s
✅ REPOSITORY	git_diff	yes		no	no	0.1s
✅ REPOSITORY	secretlint	yes		no	no	4.54s
✅ REPOSITORY	syft	yes		no	no	4.23s
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✅ REPOSITORY	trufflehog	yes		no	no	15.0s
✅ YAML	prettier	5	0	0	0	0.47s
✅ YAML	v8r	5		0	0	28.79s
✅ YAML	yamllint	5		0	0	0.48s

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