Fix double .compute() when render_points color= is a native column

[timtreis]

Summary

• render_points(color=<native_col>) was calling dd.DataFrame.compute() twice: once explicitly to load the points (render.py:747) and again inside _set_color_source_vec via a redundant get_values() call (utils.py:1077).
• Root cause: the _preloaded shortcut only fired when added_color_from_table=True. For element-native color columns the flag stayed False, so _set_color_source_vec fell back to a fresh get_values() call.
• Fix: widen the condition to col_for_color is not None and col_for_color in points_pd_with_color.columns, which is True for both native-column and table-sourced cases.

Closes #633.

Test plan

• test_render_points_native_color_column_single_compute — patches dd.DataFrame.compute to count calls and asserts exactly 1 for a native categorical color column

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.39%. Comparing base (e1f8863) to head (75434d6).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #643   +/-   ##
=======================================
  Coverage   76.39%   76.39%           
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  Files          11       11           
  Lines        3237     3237           
  Branches      759      761    +2     
=======================================
  Hits         2473     2473           
  Misses        466      466           
  Partials      298      298           

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/render.py	86.67% <100.00%> (+0.08%)	⬆️

... and 2 files with indirect coverage changes

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