Deprecated size usage in exported helpers and documentation examples

[ishaan-arora-1]

A few places still use size where ggplot2 3.4+ expects linewidth:

Exported API:

• grid_lines(color, size) in R/bayesplot-helpers.R accepts a size parameter and its @param documents it as size, but internally passes it as linewidth to element_line(). The parameter should be renamed to linewidth (with size kept as a deprecated alias for back-compat).
• The internal grid_lines_y() has the same issue.

Documentation examples:

• R/mcmc-diagnostics.R line 80: hline_at(0.5, linetype = 2, size = 0.15, color = "gray") — should use linewidth since hline_at() forwards ... to geom_hline().
• R/mcmc-scatterplots.R line 112: geom_smooth(method = "lm", se = FALSE, color = "gray20", size = .75, linetype = 2) — should use linewidth.

These produce deprecation warnings on ggplot2 >= 3.4.0.

AI Usage Policy :
This issue was found using an LLM

[ishaan-arora-1]

What do you think @jgabry?

[jgabry]

I agree! I requested one change on the PR, but otherwise looks good.