[Bug] A scene saved by the editor loses what the pack loader set up: shared builds, LOD chains, and the screen-size selection #1326

Goal

A scene the editor saves and reloads draws as it did before the save.

What happens

The editor reloads a saved scene's placements one by one, through addLODLevel and the plain mesh path, not through the pack loader. Three things the pack loader sets up are therefore lost on reload:

• the shared builds of a model placed many times ([Performance] Load each .untold of a pack once #1290): each placement gets its own copy;
• the LOD chains of multi-node models ([Performance] Automatic LOD chains for the models of a .untoldpack #1299): the parts of a model no longer switch together;
• LODComponent.selectsByScreenSize and screenSizeRadius ([Feature] Select LOD levels by the size of the entity on screen #1322): the scene serializer does not write them, so a reloaded entity selects its levels by the stored distances, which were computed for the field of view of the moment the pack was loaded.

Next steps

One change: a cache per scene load, the chain found from the model, and the two LOD fields in the serializer with a format version bump. Noted as a follow-up in docs/proposals/LargeSceneRendering.md.