Add per-channel norm support for render_images

[timtreis]

Summary

• Closes Document how to perform on-the-fly normalization for channels. #460 — render_images now accepts a list of Normalize objects for per-channel normalization
• Essential for multi-channel protein/fluorescence data with vastly different intensity ranges per channel
• The rendering pipeline already read per-channel norms from CmapParams — this change only widens validation and routes per-channel norms into the existing CmapParams creation loop

Changes

• basic.py: Widen norm type to list[Normalize] | Normalize | None, update docstring, zip norm list with cmap list
• utils.py: Accept list[Normalize] in validation, reject empty lists and non-Normalize elements
• test_render_images.py: 5 new tests (per-channel norm, backward compat, length mismatch, empty list, invalid element)

Usage

from matplotlib.colors import Normalize

norms = [
    Normalize(vmin=0, vmax=0.05, clip=True),  # dim channel
    Normalize(vmin=0, vmax=1.0, clip=True),    # full range
    Normalize(vmin=0, vmax=0.5, clip=True),    # medium
]
sdata.pl.render_images(
    channel=[0, 1, 2],
    norm=norms,
    # cmaps are optiopnal
    cmap=[plt.cm.gray] * 3,
).pl.show()

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.17%. Comparing base (367204d) to head (f12292b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata_plot/pl/utils.py	50.00%	1 Missing and 2 partials ⚠️
src/spatialdata_plot/pl/render.py	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #572      +/-   ##
==========================================
- Coverage   75.61%   75.17%   -0.44%     
==========================================
  Files          10       10              
  Lines        2908     2929      +21     
  Branches      672      681       +9     
==========================================
+ Hits         2199     2202       +3     
- Misses        428      445      +17     
- Partials      281      282       +1     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/basic.py	83.72% <100.00%> (+0.55%)	⬆️
src/spatialdata_plot/pl/render.py	86.69% <0.00%> (-0.17%)	⬇️
src/spatialdata_plot/pl/utils.py	65.32% <50.00%> (-0.85%)	⬇️

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