Add support for alternative point coloring

Add support for coloring points based on other features than class (e.g. cosine similarity and other embedding distances).

The issue names no files, tests, or entry points. Start by locating the existing point-coloring flow and reviewing how point features are represented, then identify where cosine similarity or other embedding distances would enter. Done means points can be colored by a selected non-class feature, with coverage for the supported alternatives.

Written by the indexing model from the issue text.