Add ps.no_spines() for imshow/pcolormesh/2D-histogram plots

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>

plotstyle/figures.py

@@ -88,6 +88,20 @@ def new_figure(
     return fig, axes
 
 
+def no_spines(ax: Union[Axes, np.ndarray]) -> None:
+    """Hide every spine on `ax` — pass a single Axes or an array of them.
+
+    `use()` keeps only the bottom spine visible, since most plots have a
+    meaningful x baseline. Pixel/bin-indexed plots (`imshow`, `pcolormesh`,
+    2D histograms) don't — there's no "zero" the bottom spine anchors to —
+    so call this on their Axes instead of leaving the themed bottom spine on
+    or hand-rolling `ax.spines[...].set_visible(False)`.
+    """
+    for single_ax in [ax] if isinstance(ax, Axes) else np.ravel(ax):
+        for spine in single_ax.spines.values():
+            spine.set_visible(False)
+
+
 def colorbar(mappable, ax: Axes, size: str = "5%", pad: float = 0.05, **kwargs) -> Colorbar:
     """Add a colorbar matched to `ax`'s actual on-screen size.