feat: port session 05 — Subscription Index (sublist)

Port trie-based subject matching engine (81 features, 74 tests).
Includes SubscriptionIndex with cache, wildcard matching (*/>),
queue subscription groups, reverse match, notifications, stats,
and subject validation utilities. Also adds minimal Subscription
and NatsClient stubs needed by the index.

dotnet/src/ZB.MOM.NatsNet.Server/Internal/Subscription.cs

@@ -0,0 +1,77 @@
+// Copyright 2012-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Adapted from server/client.go (subscription struct) in the NATS server Go source.
+
+namespace ZB.MOM.NatsNet.Server.Internal;
+
+/// <summary>
+/// Represents a client subscription in the NATS server.
+/// Mirrors the Go <c>subscription</c> struct from client.go.
+/// This is a minimal stub; full client integration will be added in later sessions.
+/// </summary>
+public sealed class Subscription
+{
+    /// <summary>The subject this subscription is listening on.</summary>
+    public byte[] Subject { get; set; } = [];
+
+    /// <summary>The queue group name, or null/empty for non-queue subscriptions.</summary>
+    public byte[]? Queue { get; set; }
+
+    /// <summary>The subscription identifier.</summary>
+    public byte[]? Sid { get; set; }
+
+    /// <summary>Queue weight for remote queue subscriptions.</summary>
+    public int Qw;
+
+    /// <summary>Closed flag (0 = open, 1 = closed).</summary>
+    private int _closed;
+
+    /// <summary>The client that owns this subscription. Null in test/stub scenarios.</summary>
+    public NatsClient? Client { get; set; }
+
+    /// <summary>Marks this subscription as closed.</summary>
+    public void Close() => Interlocked.Exchange(ref _closed, 1);
+
+    /// <summary>Returns true if this subscription has been closed.</summary>
+    public bool IsClosed() => Interlocked.CompareExchange(ref _closed, 0, 0) == 1;
+}
+
+/// <summary>
+/// Represents the kind of client connection.
+/// Mirrors Go's <c>clientKind</c> enum.
+/// This is a minimal stub; full implementation in later sessions.
+/// </summary>
+public enum ClientKind
+{
+    Client = 0,
+    Router = 1,
+    Gateway = 2,
+    System = 3,
+    Leaf = 4,
+    JetStream = 5,
+    Account = 6,
+}
+
+/// <summary>
+/// Minimal client stub for subscription routing.
+/// Full implementation will be added in later sessions.
+/// </summary>
+public class NatsClient
+{
+    /// <summary>The kind of client connection.</summary>
+    public ClientKind Kind { get; set; }
+
+    /// <summary>Whether this is a hub leaf node. Stub for now.</summary>
+    public virtual bool IsHubLeafNode() => false;
+}