Fix vMF deterministic sigma-point sampler

[FlorianPfaff]

Summary

• Fix the von Mises-Fisher deterministic sigma-point sampler to use the embedding-dimension mean resultant length a_d(input_dim, kappa).
• Replace the invalid iv(..., 1) usage in the sampler path by the existing a_d helper and make a_d use the scaled Bessel ratio ive for large-kappa stability.
• Correct the sigma-point layout so columns are samples in embedding coordinates.
• Add regression tests for deterministic-sample shape, unit norm, and equal-weight mean-resultant matching.

Notes

I could not run the full repository test suite in this environment because GitHub/network checkout is unavailable here. The changed Python files were syntax-checked locally before pushing through the connector.