Add reference images and fix labels visual test

Fix test_plot_dict_palette_hex_labels to use a hand-built dict
instead of make_palette_from_data, since labels elements don't
have extractable coordinates.

Add 5 CI-generated reference images for palette visual tests.
The hex_labels reference image will be generated on the next CI run.

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

Author: timtreis

tests/_images/PaletteVisual_dict_palette_hex_points.png

@@ -495,7 +495,7 @@ def test_plot_dict_palette_hex_shapes(self, sdata_blobs: SpatialData):
 
     def test_plot_dict_palette_hex_labels(self, sdata_blobs: SpatialData):
         """Visual test: hex dict palette renders labels correctly."""
-        palette = make_palette_from_data(sdata_blobs, "blobs_labels", "region", palette="okabe_ito")
+        palette = {"blobs_labels": "#E69F00"}
         sdata_blobs.pl.render_labels("blobs_labels", color="region", palette=palette).pl.show()
 
     def test_plot_dict_palette_named_colors_points(self, sdata_blobs: SpatialData):