fix(signalr): scope stream materializer to per-session actor to prevent actor leak

[Aaronontheweb]

Summary

• Root cause: SessionRegistry.MaterializeAndBindSessionAsync() called .Run(_system) for all stream materializations, creating stream stage actors under the global StreamSupervisor-0. These accumulated over the daemon's lifetime and were only torn down at full system shutdown — visible as a large burst of Flow-*-actorRefSource* actors in shutdown logs.
• Fix: Introduces SignalRSessionActor, a per-session binding actor mirroring the SlackThreadBindingActor pattern. Each actor creates Context.Materializer(namePrefix:"signalr") scoped to its own context, making all stream actors its children. When the session ends, the actor stops → materializer disposes → all stream children are automatically cleaned up.
• New actor hierarchy: /netclaw/user/signalr-gateway/{sessionId} (owned, lifecycle-bound streams) replaces orphaned actors under StreamSupervisor-0.

Changes

• SignalRSessionActor — per-session binding actor with Initializing/Active state machine, scoped materializer, pipeline reinitialization on stream termination, and PostStop cleanup
• SignalRGatewayHostingExtensions — registers signalr-gateway (GenericChildPerEntityParent) and SignalRGatewayActorKey
• SessionRegistry — rewritten to delegate pipeline lifecycle to the actor via Tell; now tracks session existence only (no stream state)
• Program.cs — wires .WithSignalRGateway() into the Akka builder
• SessionRegistryTests — rewritten to cover registry coordination behavior using a StubRequiredActor

Test plan

• All existing tests pass (dotnet test)
• dotnet slopwatch analyze — no new violations
• Start daemon, run sessions, shut down — shutdown logs should show no Flow-*-actorRefSource* burst under StreamSupervisor-0
• Session actors appear under /netclaw/user/signalr-gateway/ and stop when sessions end