Revise JAX intro lecture and add autodiff lecture

- Fix coefficient mismatch in NumPy vs JAX function comparison
- Update jax.random.PRNGKey to jax.random.key throughout
- Add code-generated figures (PRNG key splitting tree, JIT pipeline)
- Add vmap section with examples and transformation composition
- Rework gradients section as autodiff preview with forward reference
- Add autodiff lecture (adapted from lecture-jax) to TOC
- Consolidate all imports into initial cell

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jstac

lectures/_toc.yml

@@ -25,6 +25,7 @@ parts:
   - file: numba
   - file: jax_intro
   - file: numpy_vs_numba_vs_jax
+  - file: autodiff
 - caption: Working with Data
   numbered: true
   chapters: