Investigate threading and concurrency issues

While `AppKit` dispatches UI events on the main thread, `SceneKit` and `SpriteKit` seem to have their own private render threads, potentially resulting in race conditions, use-after-frees and other concurrency issues when the model is modified/accessed from different threads:

A potential solution would be to look into Swift 5's new actor classes.

The issue names no files, tests, or entry points. Start by tracing how AppKit, SceneKit, and SpriteKit access the model across threads, then investigate whether Swift actors address the reported races and use-after-frees; done should mean the concurrency risks are resolved or clearly characterized.

Written by the indexing model from the issue text.