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>

tests/test_plotstyle.py

@@ -210,6 +210,26 @@ def test_colorbar_has_no_outline():
         plt.close(fig)
 
 
+def test_no_spines_hides_all_spines_on_single_axes():
+    fig, ax = ps.new_figure("square")
+    try:
+        ax.imshow([[0, 1], [2, 3]])
+        ps.no_spines(ax)
+        assert all(not spine.get_visible() for spine in ax.spines.values())
+    finally:
+        plt.close(fig)
+
+
+def test_no_spines_hides_all_spines_on_axes_array():
+    fig, axes = ps.new_figure("square", nrows=1, ncols=2)
+    try:
+        ps.no_spines(axes)
+        for ax in axes:
+            assert all(not spine.get_visible() for spine in ax.spines.values())
+    finally:
+        plt.close(fig)
+
+
 def test_savefig_writes_requested_formats(tmp_path):
     fig, ax = ps.new_figure("square")
     ax.plot([0, 1], [0, 1])