Fill the viewport with a fixed sidebar of highlightable type-filter
icons over an independently scrolling tree, and let the viewer expand
into a full-height scene.
Key tree nodes by their index path instead of GUID, since TTS saves can
contain duplicate GUIDs. Skip the Parent back-reference in the default
viewer and stringify with a circular-reference guard.
Add a containment-tree sidebar with class tags and display labels, plus
an inspector pane backed by a viewer registry with a default viewer.
Custom per-class viewers can be registered later.
hypercross