Fix E2E test gaps and add comprehensive E2E + parity test suites

- Fix pull consumer fetch: send original stream subject in HMSG (not inbox)
  so NATS client distinguishes data messages from control messages
- Fix MaxAge expiry: add background timer in StreamManager for periodic pruning
- Fix JetStream wire format: Go-compatible anonymous objects with string enums,
  proper offset-based pagination for stream/consumer list APIs
- Add 42 E2E black-box tests (core messaging, auth, TLS, accounts, JetStream)
- Add ~1000 parity tests across all subsystems (gaps closure)
- Update gap inventory docs to reflect implementation status

tests/NATS.E2E.Tests/Infrastructure/AuthServerFixture.cs

@@ -0,0 +1,82 @@
+using NATS.Client.Core;
+using NATS.NKeys;
+
+namespace NATS.E2E.Tests.Infrastructure;
+
+public sealed class AuthServerFixture : IAsyncLifetime
+{
+    private NatsServerProcess _server = null!;
+
+    public string NKeyPublicKey { get; }
+    public string NKeySeed { get; }
+
+    public int Port => _server.Port;
+
+    public AuthServerFixture()
+    {
+        var kp = KeyPair.CreatePair(PrefixByte.User);
+        NKeyPublicKey = kp.GetPublicKey();
+        NKeySeed = kp.GetSeed();
+    }
+
+    public async Task InitializeAsync()
+    {
+        var config = $$"""
+            max_payload: 512
+
+            authorization {
+                users: [
+                    { user: "testuser", password: "testpass" },
+                    {
+                        user: "pubonly",
+                        password: "pubpass",
+                        permissions: {
+                            publish: { allow: ["allowed.>"] },
+                            subscribe: { allow: ["_INBOX.>"] }
+                        }
+                    },
+                    {
+                        user: "subonly",
+                        password: "subpass",
+                        permissions: {
+                            subscribe: { allow: ["allowed.>", "_INBOX.>"] },
+                            publish: { allow: ["_INBOX.>"] }
+                        }
+                    },
+                    { user: "limited", password: "limpass" },
+                    { nkey: "{{NKeyPublicKey}}" }
+                ]
+            }
+
+            max_subs: 5
+            """;
+
+        _server = NatsServerProcess.WithConfig(config);
+        await _server.StartAsync();
+    }
+
+    public async Task DisposeAsync()
+    {
+        await _server.DisposeAsync();
+    }
+
+    public NatsConnection CreateClient(string user, string password)
+    {
+        var opts = new NatsOpts
+        {
+            Url = $"nats://127.0.0.1:{Port}",
+            AuthOpts = new NatsAuthOpts
+            {
+                Username = user,
+                Password = password,
+            },
+        };
+        return new NatsConnection(opts);
+    }
+
+    public NatsConnection CreateClient()
+        => new(new NatsOpts { Url = $"nats://127.0.0.1:{Port}" });
+}
+
+[CollectionDefinition("E2E-Auth")]
+public class AuthCollection : ICollectionFixture<AuthServerFixture>;