[Feature]: Change notifications / reactive state

[eneshoxha]

To tie Cortex.States and Cortex.Streams together even tighter, we are adding observable change feeds.

1. Change model

public enum StateChangeKind
{
    Added,
    Updated,
    Removed
}

public readonly record struct StateChange<TKey, TValue>(
    TKey Key,
    TValue? OldValue,
    TValue? NewValue,
    StateChangeKind Kind);

2. Observable store

public interface IObservableDataStore<TKey, TValue> : IDataStore<TKey, TValue>
{
    IObservable<StateChange<TKey, TValue>> Changes { get; }
}

InMemoryStateStore implementation:

• Wrap Put / Remove to publish StateChange on a Subject<StateChange<…>>.
• Lazy-create the subject when first subscribed to avoid overhead if unused.

Usage examples:

• Streams operators can subscribe to changes and emit derived events.
• ProjectionTables can incrementally update cached projections instead of recomputing on GetAll().

Async-friendly variant:

• Allow a Channel<StateChange<…>> or extend to IAsyncEnumerable<StateChange<…>> ChangesAsync(CancellationToken) where supported.