Z-Ordering of entity collection

It would be nice to be able to change the render order of entities. Right now we have to tweak the `eyeOffset` which needs to change based on camera distance.

Start by tracing how entity collections are rendered and how eyeOffset currently affects their apparent order. Determine the intended z-order behavior independently of camera distance, then define a concrete API and verify ordering across relevant camera positions. The issue names no files or tests, so repository exploration and design discussion are required before implementation.

Written by the indexing model from the issue text.