feat(hermite-legendre): add conserving collision models

[joglekara]

Summary

• add an O(Nh + Nl) conserving BGK operator for inexpensive physical dissipation and regularization
• add a basis-native conserving Dougherty Fokker-Planck operator with exact Hermite relaxation and prediagonalized Legendre diffusion
• expose both through physics.collisions while retaining the existing hypercollisional absorbers
• document conservation, representation, and cost tradeoffs, including comparison with a batched tridiagonal solve

Validation

• Ruff format and lint checks
• 8 focused collision tests covering conservation, damping, equilibrium, and end-to-end model selection
• Hermite-Legendre streaming, split, and linear-advection regressions
• 25 affected tests passed