fix: full scanpy 1.13 compatibility layer

[timtreis]

Goal

Make spatialdata-plot fully compatible with scanpy 1.13 (the pre-release used in the py3.14-pre CI env), which reorganized its plotting internals and broke import spatialdata_plot and several render paths.

Complete scanpy-internal surface (audited across src/)

Symbol	scanpy 1.13 change	Handling
default_20/28/102	plotting.palettes → plotting.legacy.palettes	import with fallback
_add_categorical_legend	plotting._tools.scatterplots → plotting.legacy._tools...	import with fallback
add_colors_for_categorical_sample_annotation	plotting._utils → plotting.legacy._utils	import with fallback
settings._vector_friendly	removed	vector_friendly() helper, defaults to False, still honours sc.set_figure_params on older scanpy
sc.get.obs_df, settings.figdir, sc.set_figure_params, sc.pl.set_rcParams_defaults	unchanged	used directly, no change

All of it is consolidated into one version-tolerant module, _scanpy_compat, and every call site (render.py, utils.py, _color.py, _palette.py) routes through it — so the reliance on scanpy internals lives in exactly one place.

Why behaviour is unchanged

The relocated palettes/helpers are frozen copies: default_* hash identically across 1.11.4 and 1.13.0a1, and _add_categorical_legend / add_colors_... have identical signatures in the old and legacy locations. vector_friendly() reproduces scanpy's historical default.

Verification

• Symbols resolve on stable scanpy 1.11.4 (fallback) and pre-release 1.13.0a1 (new path).
• Resolved against scanpy 1.13.0a1, the full non-visual suite passes and there are zero scanpy AttributeError/ImportError (visual test_plot_* only run on the GH runner). CI's py3.14-pre is the authoritative visual check.

Notes

Follow-up to the merged #759 (palettes-only). py3.14-pre is continue-on-error (non-gating); this is scanpy-1.13 readiness.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.63%. Comparing base (5627c9b) to head (ce185c0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #760      +/-   ##
==========================================
+ Coverage   79.60%   79.63%   +0.03%     
==========================================
  Files          18       18              
  Lines        4657     4665       +8     
  Branches     1032     1032              
==========================================
+ Hits         3707     3715       +8     
  Misses        599      599              
  Partials      351      351              

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/_color.py	70.03% <100.00%> (-0.05%)	⬇️
src/spatialdata_plot/pl/_palette.py	85.31% <100.00%> (ø)
src/spatialdata_plot/pl/_scanpy_compat.py	100.00% <100.00%> (ø)
src/spatialdata_plot/pl/render.py	89.40% <100.00%> (-0.02%)	⬇️
src/spatialdata_plot/pl/utils.py	78.97% <100.00%> (-0.04%)	⬇️

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