Add SO3 uniform distribution

[FlorianPfaff]

Summary

• Add SO3UniformDistribution for Haar-uniform rotations represented as scalar-last unit quaternions (x, y, z, w) on the canonical upper S^3 hemisphere.
• Provide constant pdf/ln_pdf, backend-compatible canonical sampling, SO(3) geodesic distance, quaternion-to-rotation-matrix conversion, and explicit unavailable mean/mode behavior.
• Export the new distribution and add focused tests for inheritance, density, sampling, distance, rotation matrices, and undefined mean/mode access.

Validation

• python -m unittest tests.distributions.test_so3_uniform_distribution
• $env:PYRECEST_BACKEND='numpy'; python -m pytest -q tests/distributions/test_so3_uniform_distribution.py
• $env:PYRECEST_BACKEND='jax'; python -m pytest -q tests/distributions/test_so3_uniform_distribution.py
• $env:PYRECEST_BACKEND='pytorch'; python -m pytest -q tests/distributions/test_so3_uniform_distribution.py
• python -m unittest tests.distributions.test_so3_uniform_distribution tests.distributions.test_hyperspherical_uniform_distribution
• python -m black --check src/pyrecest/distributions/so3_uniform_distribution.py tests/distributions/test_so3_uniform_distribution.py
• python -m ruff check src/pyrecest/distributions/so3_uniform_distribution.py tests/distributions/test_so3_uniform_distribution.py src/pyrecest/distributions/__init__.py
• python -m pylint src/pyrecest/distributions/so3_uniform_distribution.py tests/distributions/test_so3_uniform_distribution.py
• python -m mypy --ignore-missing-imports src/pyrecest/distributions/so3_uniform_distribution.py tests/distributions/test_so3_uniform_distribution.py

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	14.62s
✅ JSON	prettier	2	0	0	0	0.44s
✅ JSON	v8r	2		0	0	2.37s
✅ MARKDOWN	markdownlint	21	0	0	0	1.17s
✅ MARKDOWN	markdown-table-formatter	21	0	0	0	0.35s
✅ PYTHON	bandit	437		0	0	6.83s
✅ PYTHON	black	437	0	0	0	10.41s
✅ PYTHON	flake8	437		0	0	3.87s
✅ PYTHON	isort	437	0	0	0	0.83s
✅ PYTHON	mypy	437		0	0	5.8s
✅ PYTHON	pylint	437		0	0	138.9s
✅ PYTHON	ruff	437	0	0	0	0.08s
✅ REPOSITORY	checkov	yes		no	no	24.84s
✅ REPOSITORY	gitleaks	yes		no	no	8.15s
✅ REPOSITORY	git_diff	yes		no	no	0.07s
✅ REPOSITORY	secretlint	yes		no	no	8.21s
✅ REPOSITORY	syft	yes		no	no	6.64s
✅ REPOSITORY	trivy-sbom	yes		no	no	3.7s
✅ REPOSITORY	trufflehog	yes		no	no	20.28s
✅ YAML	prettier	5	0	0	0	0.54s
✅ YAML	v8r	5		0	0	5.28s
✅ YAML	yamllint	5		0	0	0.54s

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[github-actions]

Test Results

    12 files  +    12      12 suites  +12   4h 6m 14s ⏱️ + 4h 6m 14s
 1 233 tests + 1 233   1 233 ✅ + 1 233      0 💤 ±    0  0 ❌ ±0 
15 160 runs  +15 160  11 624 ✅ +11 624  3 536 💤 +3 536  0 ❌ ±0 

Results for commit 817e6f9. ± Comparison against base commit ddae39b.

♻️ This comment has been updated with latest results.