feat: add sort_order to spatial_scatter

[Georicl]

Description

Add sort_order=True to pl.spatial_scatter so higher continuous color values are drawn on top. Sort independently within each panel after subsetting, draw missing values first, and preserve observation order for equal values. sort_order=False retains the previous drawing order; categorical and uncolored plots retain their order.

Keep per-point collection styles aligned with the reordered coordinates, including Matplotlib aliases, cycled style sequences, scalar colors, and dash specifications. Document the new default in the API and release notes.

How has this been tested?

• Spatial plotting tests: 133 passed, 1 skipped by the existing macOS marker.
• New style regression tests reproduced 32 failures before the style fix; all pass after it.
• Pixel checks cover high-expression overlap for ordinary scatter, circles, squares, and hexagons.
• Regression coverage includes .obs, .X, layers, raw, missing and tied values, categorical colors, multiple libraries and panels, cropping, outlines, per-point styles, and patch offsets.
• All applicable repository pre-commit checks pass.
• Full Sphinx HTML build passes with warnings treated as errors; verified the generated scatter signature and parameter documentation, and that segmentation does not expose sort_order.

Closes

Closes #824

[codecov]

Codecov Report

❌ Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.57%. Comparing base (8286274) to head (d1fae25).

Files with missing lines	Patch %	Lines
src/squidpy/pl/_spatial_utils.py	96.42%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1291      +/-   ##
==========================================
+ Coverage   78.44%   78.57%   +0.13%     
==========================================
  Files          63       63              
  Lines        9532     9559      +27     
  Branches     1594     1602       +8     
==========================================
+ Hits         7477     7511      +34     
+ Misses       1489     1487       -2     
+ Partials      566      561       -5     

Files with missing lines	Coverage Δ
src/squidpy/pl/_spatial.py	95.23% <100.00%> (+2.38%)	⬆️
src/squidpy/pl/_spatial_utils.py	80.40% <96.42%> (+1.83%)	⬆️

... and 1 file with indirect coverage changes

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