feat: add SiteStreamGrpcServer with Channel<T> bridge and stream limits
- Define ISiteStreamSubscriber interface for decoupling from SiteRuntime
- Implement SiteStreamGrpcServer (inherits SiteStreamServiceBase) with:
- Readiness gate (SetReady)
- Max concurrent stream enforcement
- Duplicate correlationId replacement (cancels previous stream)
- StreamRelayActor creation per subscription
- Bounded Channel<SiteStreamEvent> bridge (1000 capacity, drop-oldest)
- Clean teardown: unsubscribe, stop actor, remove tracking entry
- Identity-safe cleanup using ConcurrentDictionary.TryRemove(KeyValuePair)
to prevent replacement streams from being removed by predecessor cleanup
- 7 unit tests covering reject-not-ready, max-streams, duplicate cancel,
cleanup-on-cancel, subscribe/remove lifecycle, event forwarding
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