SE-0475: Transactional Observation of Values

[John_McCall]

@KeithBauerANZ

As discussed in the ProgressReporter thread, withObservationTracking inherently creates a window where concurrent changes are not tracked. Observation really wants to be able to guarantee that Observable types are either global-actor-isolated or non-Sendable, which the type system can't represent.

This looks like a real problem in the current implementation, but it isn't inherent to the design and should be fixable.