Add legend placement tests and use CI-generated baselines

Replace local macOS baselines with CI-generated (Linux) images for
the 5 existing channel legend tests.  Add 2 new visual tests for
legend positioning (upper left, lower right) to verify legend_loc
is respected by the channel legend.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: timtreis

tests/_images/ChannelsAsCategories_channels_as_categories_legend_lower_right.png

@@ -524,6 +524,16 @@ def test_plot_channels_as_categories_with_cmap_list(self, sdata_blobs: SpatialDa
             channels_as_categories=True,
         ).pl.show()
 
+    def test_plot_channels_as_categories_legend_upper_left(self, sdata_blobs: SpatialData):
+        sdata_blobs.pl.render_images(element="blobs_image", channel=[0, 1], channels_as_categories=True).pl.show(
+            legend_loc="upper left"
+        )
+
+    def test_plot_channels_as_categories_legend_lower_right(self, sdata_blobs: SpatialData):
+        sdata_blobs.pl.render_images(element="blobs_image", channel=[0, 1], channels_as_categories=True).pl.show(
+            legend_loc="lower right"
+        )
+
 
 class TestChannelsAsCategoriesNonVisual:
     """Non-visual tests for channels_as_categories edge cases."""