Fix multi-channel compositing and add per-channel norm support#451

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timtreis wants to merge 11 commits intomainfrom

bugfix/issue450-feature-pca-aggregation-for-n-4-channels-in-render_images

Commits on Mar 19, 2026

Unify multi-channel compositing to additive blending and add per-channel norm

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Remove unused percentiles_for_norm field and deprecation alias

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Update CI-generated baseline images for additive blending compositing

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Use signal-based compositing for correct blending with any colormap

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Add tests for 5-channel compositing and per-channel normalization

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Address review: deduplicate cmap(0) call, add error tests, trim warning

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Use Color.get_alpha_as_float() instead of comparing internal hex string

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Reject tuple norms with clear error, standardize na_is_transparent guards

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Add CI-generated baseline images for signal-based compositing tests

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Clean up comments: remove AI-style symbols and reword unclear comments

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Add empty-layers guard to _additive_blend, clarify scalar_norm comment

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