Refactor element state model to support reactive framework integration

[spaansba]

Summary

• Replaces mutable ForesightElementData with an immutable ForesightElementState snapshot that is swapped on every change, enabling compatibility with React's useSyncExternalStore and Vue's shallowRef
• Introduces subscribe / getSnapshot on ForesightRegisterResult so framework adapters can reactively bind to element state without polling or manual event wiring
• Splits internal bookkeeping (ForesightElementInternal) from the public state surface, using a DerivedMapView to prevent leaking internals through registeredElements
• Flattens the nested callbackInfo object into top-level state fields (isPredicted, isCallbackRunning, hitCount, durationMs, status, error, etc.)
• Updates devtools, demo pages, and existing hooks (useForesight) to consume the new state shape
• Adds tests for subscribe/getSnapshot lifecycle, DerivedMapView, re-registration, and related edge cases

This is the first step towards creating pre-made framework adaptors