Route plain density through the continuous shade path

For ``color=None``, the dispatch in ``_render_points`` was sending the
aggregate to ``_ds_shade_categorical``, which uses ``color_vector[0]``
as a single hex color and only modulates alpha across counts. That
collapses any sequential colormap (``viridis`` and friends) to a flat
hue and makes a "density" plot look like a uniform smudge with low
opacity.

Plain density now routes through ``_ds_shade_continuous`` so the
configured cmap is used as a real intensity gradient over the count
aggregate. ``density_how`` ("linear" / "log" / "cbrt" / "eq_hist")
finally produces visibly different results.

The categorical density branch is unchanged: per-category color keys
still pass through ``_ds_shade_categorical`` and modulate alpha per
gene, which is the correct behaviour for "where does each category
concentrate".

Author: timtreis

src/spatialdata_plot/pl/render.py

@@ -1091,9 +1091,13 @@ def _render_points(
             color_vector = np.asarray([_hex_no_alpha(c) for c in color_vector])
 
         shade_how = render_params.density_how if render_params.density else "linear"
+        # Plain density (no color column) must use the user-facing cmap as a sequential
+        # gradient over counts; the categorical path collapses to a single color and only
+        # modulates alpha, which renders as a flat hue regardless of density.
+        plain_density = render_params.density and col_for_color is None
 
         nan_shaded = None
-        if color_by_categorical or col_for_color is None:
+        if not plain_density and (color_by_categorical or col_for_color is None):
             shaded = _ds_shade_categorical(
                 agg,
                 color_key,