Pin jax in the numpy_vs_numba_vs_jax install cell (#621)

The workflow pin from #620 protects the build but not the reader. In CI the
cell is a no-op because jax 0.11.0 is already installed, so pip treats the
unpinned requirement as satisfied. In Colab nothing pre-satisfies it: a
reader executing this notebook today resolves jax 0.11.1, then reaches the
CPU-pinned lax.fori_loop and lax.scan cells at n = 10,000,000.

jax 0.11.1 regresses XLA:CPU execution quadratically -- measured on linux
x86_64, doubling n multiplies runtime by ~4.1 (fori) and ~4.0 (scan). At
n=400,000 it is 105s against 0.005s under 0.11.0; extrapolated to the
lecture's n=10,000,000 that is roughly 18 hours, which a reader experiences
as a hang.

Pinned to ==0.11.0 rather than !=0.11.1 deliberately: the regression is
still present on jax main (nightly 0.11.2.dev20260819 measured at 96% of
0.11.1's time), so an exclusion would admit a likely-broken 0.11.2 without
warning. This matches the exact version the six workflows install, so
readers now run what CI runs.

The other two jax cells in this repo are untouched: jax_intro.md loops to
n=20 and autodiff.md has no lax loop, so neither reaches the regime.

Lifting this pin is tracked alongside the workflow pins in
QuantEcon/workspace-lectures#49.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: mmcky

lectures/numpy_vs_numba_vs_jax.md

@@ -45,7 +45,7 @@ In addition to what's in Anaconda, this lecture will need the following librarie
 ---
 tags: [hide-output]
 ---
-!pip install quantecon jax
+!pip install quantecon "jax==0.11.0"
 ```
 
 ```{include} _admonition/gpu.md