Bug: transfunc has inconsistent/broken behavior across render methods

[timtreis]

Description

transfunc (passed via **kwargs) has inconsistent and partially broken behavior across the three element rendering methods:

render_points — correct

• Applied when color_source_vector is None (continuous data)
• Receives raw numeric values before colormap mapping
• Use case: np.log1p, np.sqrt etc. on expression values

render_shapes — inverted condition (bug)

• Applied when values_are_categorical is True (render.py ~line 396)
• At that point color_vector is a pd.Categorical of hex color strings like "#FF0000FF"
• Calling a numeric transform on hex strings is nonsensical and will either crash or silently corrupt the color vector
• The condition should be flipped to match points: apply only for continuous data

render_labels — dead code

• Extracted from kwargs and stored in LabelsRenderParams.transfunc
• _render_labels() never reads render_params.transfunc
• Parameter is silently accepted but has zero effect

Expected behavior

All three methods should match render_points: apply transfunc to raw numeric values when coloring by continuous data, and ignore it for categorical data.

Found during

Docstring & parameter audit for #463.