Bessel

Numerically stable scaled modified Bessel functions for state-space GPs.

Leaf module imported by both sde.py and kernels.py.

See plans/2026-04-21-state-space-gps-design.md §Bessel stability.

Three regimes are dispatched via nested jax.lax.cond so that only the taken branch contributes under jit and grad:

• c > truncation_order : forward recurrence seeded by i0e/i1e
• 1e-8 <= c <= truncation_order : Miller's downward recurrence rescaled by i0e(c)
• c < 1e-8 : power-series Ĩ_k(c) ≈ (c/2)^k / k! · e^{-c}