Function supportsScalarColormap

Whether scalar-colormap support for a node type is end-to-end wired (loader → projection → geometry storage → shader scalar source → material define).

• GSplats use the always-present amplitude (texel0.w of the splat texture) as the scalar source, so colormap mode is supported unconditionally.
• Points carry their scalar in texel2.x of the point texture, which exists in the FIXED 3-texel layout whether or not the dataset has scalars (0.0 identity fill) — so presence is knowable only from the userData.hasScalars stamp the texel writers set from data.scalars !== undefined (createPointsGeometry and the gpu-buffer-pool points adapter). This is the texture-storage analog of the old hasAttribute('aScalar') probe, driven by the same signal that used to bind the attribute.
• Lines carry their per-endpoint scalars in texel5.xy of the line texture — same fixed-layout situation as points, so presence rides the identical userData.hasScalars stamp (stampLinePresenceFlags in line-geometry.ts, called by every texel-write path).

Use this to fail-closed: if false, the caller should NOT enable USE_COLORMAP and should log a warning so the user understands why a metadata-authored colormap did not take effect.