Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper) #699

Summary

Color / cmap / norm resolution is scattered and re-implemented across render.py and utils.py, with genuine inconsistencies (not just duplication) — including a colorbar-vs-pixels mismatch and a private-matplotlib-API poke. Unifying it removes a recurring bug class and shrinks every render function.

1. CmapParams is a dumb data bag; callers re-implement its safe handling

render_params.py CmapParams carries cmap, norm, na_color, cmap_is_default but no behavior, so every consumer re-does the same operations:

• "copy the stateful Normalize before use" is hand-written 5× (render.py:705, 1231, 1765, 1862). Forgetting it risks cross-channel/cross-call mutation.
• cmap-alpha is applied by poking matplotlib's private _lut (render.py:1803-1804) — fragile across matplotlib versions and against the "no private APIs of other packages" guideline.
• na_color → hex conversions and isinstance(na_color, Color) guards are scattered ~10× (utils.py:1411/1468/1898/..., render.py:630/1318).

Fix: give CmapParams methods — fresh_norm(), cmap_with_alpha(alpha), is_user_cmap, na_hex. Behavior-preserving; spot-check the 1-channel baseline for the cmap-alpha change.

2. Continuous→RGBA normalization reimplemented 4–6× with drift

The "NaN-safe normalize then cmap(norm(.))" logic exists in _map_color_seg (utils.py:1510-1514, 1567-1571), _get_collection_shape Case B (render.py:650-669) and again at 683-691. They have already drifted: _map_color_seg uses ~np.isnan, _get_collection_shape uses np.isfinite (differ on ±inf); the vmin==vmax guard exists in one but not the other. The colorbar span is derived independently from the pixel span — so the colorbar can disagree with the rendered pixels (the #687/#688 churn class).

Fix: a single resolve_continuous_norm(values, cmap_params) -> Normalize that feeds both the pixels and the colorbar ScalarMappable, plus _continuous_to_rgba(values, cmap_params).

3. The four _render_images blend branches collapse to one helper

Paths 2A/2B/2C/2D (render.py:~1877-2015) are six copies of stack → reduce → (clip) with three different reductions. Collapse to one _composite_channels(layers, channel_cmaps, alpha). (The averaging-vs-additive inconsistency in this same code is tracked separately in findings/multichannel-compositing-bugs.md; unifying the helper is the structural enabler.)

Sequencing / risk

Land the safe foundations first — CmapParams methods (#1) — then resolve_continuous_norm (#2), then the composite helper (#3). Effort: medium. Risk: low for the CmapParams methods (no baseline impact); medium for the norm/composite unification (CI baseline regen). #2 also fixes the colorbar-vs-pixels mismatch, so it's correctness, not just cleanup.

Part of a maintainability/refactor audit of main.