feat(sessions): add SessionEventDistributor (pump + per-subscriber fan-out skeleton)

src/ZB.MOM.WW.MxGateway.Server/Sessions/IEventSubscriberLease.cs

@@ -0,0 +1,18 @@
+using System.Threading.Channels;
+using ZB.MOM.WW.MxGateway.Contracts.Proto;
+
+namespace ZB.MOM.WW.MxGateway.Server.Sessions;
+
+/// <summary>
+///     A registration lease into a <see cref="SessionEventDistributor"/>. Exposes the
+///     subscriber's own <see cref="ChannelReader{T}"/> of fanned events. Disposing the
+///     lease unregisters the subscriber and completes its channel without disturbing the
+///     pump or other subscribers.
+/// </summary>
+public interface IEventSubscriberLease : IDisposable
+{
+    /// <summary>
+    /// Gets the reader for this subscriber's fanned event channel.
+    /// </summary>
+    ChannelReader<MxEvent> Reader { get; }
+}