[Pitch] Observation performance and modeling improvements

Observation is now the model layer of most new SwiftUI code, and it is increasingly used outside SwiftUI: in middleware, in synchronization between models, and on constrained targets. That breadth exposes costs and gaps the original design couldn't anticipate. This proposal addresses them as one set of additive changes with a single goal: the cost of observation should track what is actually observed, and the shapes of model people want to write should be observable. For the readers who nee...