Image- and bin-indexed plots have no meaningful x baseline, so the
themed bottom spine kept by use() doesn't apply to them. Adds an
explicit opt-in to disable all spines on such Axes, documents it in
plotstyle/CLAUDE.md, and updates the showcase notebook's colormap and
multi-panel examples to use it.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
