[inventory_dynamics] Update code to JAX and latest style guide

Convert the lecture from Numba (jitclass / @jit(parallel=True)) to JAX:

- Replace the `Firm` jitclass with a `NamedTuple` and pure update functions.
- Use `jax.lax.scan` for the inner time loop and explicit `jax.random` keys.
- Vectorize the Monte Carlo draw loops with `jax.vmap` (including the
  50,000-draw marginal-distribution cell, which as a Python loop over a
  jitted call ran ~28s on GPU; vmapped it is ~0.3s).
- Use `sample.min()/.max()` in `plot_kde` instead of Python `min()/max()`,
  which iterate a JAX array element-by-element (~4s per call).
- Fix a hidden cross-cell dependency (`sim_length - 1` -> `2 * T`).
- Give the two exercise-solution helpers distinct names so they no longer
  shadow each other.
- Minor typo, terminology, and sentence-case title fixes.

Whole-lecture execution drops from ~57s to ~5s on an RTX 4080 with identical
numerical output.

Co-authored-by: Humphrey Yang <39026988+HumphreyYang@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: 3 people