[ar1_turningpts] Build breaks under arviz 1.x: figsize kwarg on az.plot_trace

[jstac]

Problem

ar1_turningpts.md calls

az.plot_trace(trace, figsize=(17, 6))

Under arviz 1.x (the new arviz_plots backend), plot_trace no longer accepts a figsize keyword and raises:

ValueError: Keyword arguments ['figsize'] have been passed as **kwargs but
have no active aesthetic mapped to them.

This causes a CellExecutionError and fails the build. CI pip-installs the latest arviz, so the failure appears whenever the lecture re-executes.

Why it isn't already red on main

The build uses execution_mode='cache'. The lecture's cached outputs were produced under arviz 0.x, so main stays green only until the cache is invalidated (any edit to the lecture, or a cache clear). It's a latent breakage.

Scope

Verified against arviz 1.2.0 that az.plot_trace(idata) (no figsize), az.summary(..., round_to=...), and both posterior['rho'] and posterior.rho access still work — only the figsize kwarg is the problem.

The same call existed in ar1_bayes.md and is being fixed in #927. A repo-wide grep shows ar1_turningpts.md is the only other lecture passing figsize to an arviz plotting function (bayes_nonconj.md calls plot_trace without it).

Fix

Drop figsize from the az.plot_trace call in ar1_turningpts.md (arviz uses its default size, which is also more consistent with the figure styleguide).