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.Server.Tests/Internal/InternalDsParityBatch2Tests.cs

@@ -0,0 +1,91 @@
+using System.Text;
+using NATS.Server.Internal.Avl;
+using NATS.Server.Internal.Gsl;
+using NATS.Server.Internal.SubjectTree;
+using NATS.Server.Internal.SysMem;
+using NATS.Server.Internal.TimeHashWheel;
+
+namespace NATS.Server.Tests.Internal;
+
+public class InternalDsParityBatch2Tests
+{
+    [Fact]
+    public void SubjectTreeHelper_IntersectGSL_matches_interested_subjects_once()
+    {
+        var tree = new SubjectTree<int>();
+        tree.Insert("foo.bar"u8.ToArray(), 1);
+        tree.Insert("foo.baz"u8.ToArray(), 2);
+        tree.Insert("other.subject"u8.ToArray(), 3);
+
+        var sublist = new GenericSubjectList<int>();
+        sublist.Insert("foo.*", 1);
+        sublist.Insert("foo.bar", 2); // overlap should not duplicate callback for same subject
+
+        var seen = new HashSet<string>(StringComparer.Ordinal);
+        SubjectTreeHelper.IntersectGSL(tree, sublist, (subject, _) =>
+        {
+            seen.Add(Encoding.UTF8.GetString(subject));
+        });
+
+        seen.Count.ShouldBe(2);
+        seen.ShouldContain("foo.bar");
+        seen.ShouldContain("foo.baz");
+    }
+
+    [Fact]
+    public void SubjectTree_Dump_outputs_node_and_leaf_structure()
+    {
+        var tree = new SubjectTree<int>();
+        tree.Insert("foo.bar"u8.ToArray(), 1);
+        tree.Insert("foo.baz"u8.ToArray(), 2);
+
+        using var sw = new StringWriter();
+        tree.Dump(sw);
+        var dump = sw.ToString();
+
+        dump.ShouldContain("NODE");
+        dump.ShouldContain("LEAF");
+        dump.ShouldContain("Prefix:");
+    }
+
+    [Fact]
+    public void SequenceSet_Encode_supports_destination_buffer_reuse()
+    {
+        var set = new SequenceSet();
+        set.Insert(1);
+        set.Insert(65);
+        set.Insert(1024);
+
+        var buffer = new byte[set.EncodeLength() + 32];
+        var written = set.Encode(buffer);
+        written.ShouldBe(set.EncodeLength());
+
+        var (decoded, bytesRead) = SequenceSet.Decode(buffer.AsSpan(0, written));
+        bytesRead.ShouldBe(written);
+        decoded.Exists(1).ShouldBeTrue();
+        decoded.Exists(65).ShouldBeTrue();
+        decoded.Exists(1024).ShouldBeTrue();
+    }
+
+    [Fact]
+    public void HashWheelEntry_struct_exposes_sequence_and_expiration()
+    {
+        var entry = new HashWheel.HashWheelEntry(42, 99);
+        entry.Sequence.ShouldBe((ulong)42);
+        entry.Expires.ShouldBe(99);
+    }
+
+    [Fact]
+    public void SystemMemory_returns_positive_memory_value()
+    {
+        SystemMemory.Memory().ShouldBeGreaterThan(0);
+    }
+
+    [Fact]
+    public void SimpleSubjectList_works_with_empty_marker_values()
+    {
+        var list = new SimpleSubjectList();
+        list.Insert("foo.bar", new SimpleSublistValue());
+        list.HasInterest("foo.bar").ShouldBeTrue();
+    }
+}

tests/NATS.Server.Tests/Internal/InternalDsPeriodicSamplerParityTests.cs

@@ -0,0 +1,37 @@
+using System.Reflection;
+using Microsoft.Extensions.Logging.Abstractions;
+using NATS.Server.Monitoring;
+
+namespace NATS.Server.Tests.Internal;
+
+public class InternalDsPeriodicSamplerParityTests
+{
+    [Fact]
+    [SlopwatchSuppress("SW004", "Test must observe a real 1-second CPU sampling timer tick; wall-clock elapsed time is the observable under test")]
+    public async Task VarzHandler_uses_periodic_background_cpu_sampler()
+    {
+        var options = new NatsOptions { Host = "127.0.0.1", Port = 0 };
+        var server = new NatsServer(options, NullLoggerFactory.Instance);
+        using var cts = new CancellationTokenSource();
+        _ = server.StartAsync(cts.Token);
+        await server.WaitForReadyAsync();
+
+        try
+        {
+            using var handler = new VarzHandler(server, options, NullLoggerFactory.Instance);
+            var field = typeof(VarzHandler).GetField("_lastCpuSampleTime", BindingFlags.NonPublic | BindingFlags.Instance);
+            field.ShouldNotBeNull();
+
+            var before = (DateTime)field!.GetValue(handler)!;
+            await Task.Delay(TimeSpan.FromMilliseconds(1200));
+            var after = (DateTime)field.GetValue(handler)!;
+
+            after.ShouldBeGreaterThan(before);
+        }
+        finally
+        {
+            await cts.CancelAsync();
+            server.Dispose();
+        }
+    }
+}