Add relaxed SO(3) S3F prediction helpers

[FlorianPfaff]

Summary

• add pyrecest.filters.relaxed_s3f_so3 for SO(3)/S3+ x R3 relaxed S3F prediction
• port the reusable S3R3 cell-statistics and prediction path from the S3F-Improved prototype while dropping benchmark/reporting code
• document the current method="local_tangent_samples" approximation explicitly as a numerical local-cell approximation, not exact S3+ Voronoi-cell integration
• reuse existing SO(3) helper functions from pyrecest.distributions._so3_helpers

Tests

• added focused unit tests for PSD covariance inflation, cache reuse, mass conservation, baseline/R1/R1+R2 dispatch, quaternion rotation, antipodal SO(3) distance, and validation errors
• locally syntax-checked the added files with python -m py_compile

Notes

The branch intentionally mirrors the existing relaxed_s3f_circular pattern and does not change StateSpaceSubdivisionFilter internals.