Linear EKF Dynamax Backend Refactor

[keeminlee]

Refactors EKS to use the Dynamax EKF backend across linear and pupil pipelines.

• Adds time-varying observation noise (Rₜ) from ensemble variances, shared cropping/R builders in eks.utils, and a tolerance-based jitted optimizer with blockwise s tuning
• The pupil path now uses AR(1) parameters (diameter/COM) with a stable logistic reparam and EKF filter/smoother
• Removing custom JAX forward/backward code improves stability, maintainability, and speed with Dynamax backend
• Minor API tidy-ups: always pass prebuilt Rₜ; numeric scalars use numbers.Real; handle None smooth params robustly.

Next up: Nonlinear projection implementation for calibrated multi-view setups (from .toml calibration file)