[kalman_2] Update to JAX

[xuanguang-li]

This pull request improves the lectures/kalman_2.md file, focusing on code modernization, clarity, and consistency.
The main updates include:

• Refactored the code to use jax for array creation, mathematical operations, and simulation functions. Some numpy code remains to avoid data type–related warnings.
• Improved the logic of the Kalman filter implementation, including clarifying the meaning of the variable x_hat and handling the initial states more carefully.
• Standardized parameter names in worker creation and simulation functions — for example, replacing hhat_0 and uhat_0 with μ_h and μ_u, and defining hhat_0 and uhat_0 as the priors.
• Added a seed parameter to all simulation calls.
• Fixed typos and added explanatory comments for better readability.

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[mmcky]

thanks @xuanguang-li. I have just merged in the latest main and the preview is now showing kalman_2 as the change.

[xuanguang-li]

Thanks @mmcky. I noticed a warning right below the import cell, saying.

Matplotlib is building the font cache; this may take a moment.

Should we hide this output or leave it there?