Add SO(3) quaternion grid transition helper

[FlorianPfaff]

Summary

• add so3_right_multiplication_grid_transition(...) for scalar-last SO(3) quaternion grids
• add quaternion_grid_transition_density(...) as a descriptive alias
• export both helpers from pyrecest.filters
• add focused tests for normalization, identity/nonzero increments, tangent-vs-quaternion increments, antipodal invariance, and input validation

Notes

This ports the reusable quaternion-grid transition-density primitive from the S3F-Improved dynamic-pose benchmark into PyRecEst. The helper returns an SdHalfCondSdHalfGridDistribution compatible with HyperhemisphericalGridFilter.predict_nonlinear_via_transition_density(...) and reuses PyRecEst's existing SO(3) quaternion helper functions.

Testing

Not run locally in this environment; patch is covered by tests/filters/test_so3_grid_transition.py.