Attach a color (hex string or RGB/RGBA tuple, canonicalized to RGBA) to
the evaluated result of target. Metadata rides beside the geometry: the
evaluator re-tags the shared target materialization with an independent
handle before coloring, so plain consumers of the same subtree stay
metadata-free.
CSG IR — Constructive Solid Geometry as an Intermediate Representation.
Build a content-addressed DAG of primitives, booleans, and transforms; parameterize via named expression bindings; evaluate against the active kernel with subtree-level cache reuse for incremental parametric edits.