Replace deprecated tic/toc timers with qe.Timer in kesten_processes

[devthedevil]

Summary

Closes #329.

quantecon is deprecating the Matlab-like tic/tac/toc timers in favour of the Timer context manager (QuantEcon/QuantEcon.py#833). lectures/kesten_processes.md was the only lecture in this repo still using the old API.

Change

• Dropped the now-unused from quantecon import tic, toc import (the lecture already does import quantecon as qe).
• Replaced all six tic() / toc() pairs wrapping a single timed call with the equivalent with qe.Timer(): context manager, e.g.:

# before
tic()
data = generate_cross_section(firm).block_until_ready()
toc()

# after
with qe.Timer():
    data = generate_cross_section(firm).block_until_ready()

This is a purely mechanical, behavior-preserving swap — qe.Timer() prints elapsed time on exit by default, matching toc()'s existing behavior.

Testing

This is a documentation/lecture source file (no associated unit tests). Verified by inspecting the quantecon.util.timing.Timer context-manager implementation upstream to confirm with qe.Timer(): is a drop-in replacement for the tic()/toc() pair used here.

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[mmcky]

@devthedevil I don't think this is correct unless we update variables elsewhere. Please review

[jstac]

Thanks @devthedevil! I pushed a small maintainer fix (c7310da) alongside the timer swap.

The diff had also changed one line in the first generate_cross_section_lax:

-        s = jnp.where(s < s_bar, e_t, a_t * s + b_t)
+        s = jnp.where(s < s_bar, e, a * s + b)

In that first version, the inner update_cross_section binds a, b, e (via jax.tree.map(jnp.exp, (a, b, e))), so e_t/a_t/b_t are undefined there — it would have raised a NameError when the lecture executes during the build. (The e_t/a_t/b_t in the second generate_cross_section_lax are fine, since that one defines a_t, b_t, e_t = a[t], b[t], e[t] — I left it untouched.)

Otherwise the tic/toc → qe.Timer() change looks great. This just needs a maintainer to approve the CI workflow run.

[jstac]

@mmcky should be good to merge when ready — the tic/toc → qe.Timer() swap plus the one-line fix (c7310da) for the undefined e_t/a_t/b_t in the first generate_cross_section_lax. Just needs a maintainer to approve the CI workflow run and confirm it's green.