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/JetStreamServerFixture.cs

@@ -0,0 +1,45 @@
+using NATS.Client.Core;
+
+namespace NATS.E2E.Tests.Infrastructure;
+
+public sealed class JetStreamServerFixture : IAsyncLifetime
+{
+    private NatsServerProcess _server = null!;
+    private string _storeDir = null!;
+
+    public int Port => _server.Port;
+
+    public async Task InitializeAsync()
+    {
+        _storeDir = Path.Combine(Path.GetTempPath(), "nats-e2e-js-" + Guid.NewGuid().ToString("N")[..8]);
+        Directory.CreateDirectory(_storeDir);
+
+        var config = $$"""
+            jetstream {
+                store_dir: "{{_storeDir}}"
+                max_mem_store: 64mb
+                max_file_store: 256mb
+            }
+            """;
+
+        _server = NatsServerProcess.WithConfig(config);
+        await _server.StartAsync();
+    }
+
+    public async Task DisposeAsync()
+    {
+        await _server.DisposeAsync();
+
+        if (_storeDir is not null && Directory.Exists(_storeDir))
+        {
+            try { Directory.Delete(_storeDir, recursive: true); }
+            catch { /* best-effort cleanup */ }
+        }
+    }
+
+    public NatsConnection CreateClient()
+        => new(new NatsOpts { Url = $"nats://127.0.0.1:{Port}" });
+}
+
+[CollectionDefinition("E2E-JetStream")]
+public class JetStreamCollection : ICollectionFixture<JetStreamServerFixture>;