Fix variable naming and add timing comparison to IFP EGM lecture (#746)

- Standardize function signatures: solve_model and solve_model_numpy now use (c_init, a_init) order consistently
- Fix internal variable naming: use c_in/c_out and a_in/a_out for iteration variables
- Update β from 0.96 to 0.94 to support higher interest rates (up to 6%)
- Adjust interest rate ranges to [0, 0.05] to avoid instability
- Add new Timing subsection comparing NumPy vs JAX performance
- Simplify notation: a^e_{ij} → a_{ij} throughout
- Improve variable naming consistency: c_vals/ae_vals → c_vec/a_vec, c_in/a_in → c_out/a_out

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Co-authored-by: Claude <noreply@anthropic.com>

Author: jstac