feat: port session 07 — Protocol Parser, Auth extras (TPM/certidp/certstore), Internal utilities & data structures

Session 07 scope (5 features, 17 tests, ~1165 Go LOC):
- Protocol/ParserTypes.cs: ParserState enum (79 states), PublishArgument, ParseContext
- Protocol/IProtocolHandler.cs: handler interface decoupling parser from client
- Protocol/ProtocolParser.cs: Parse(), ProtoSnippet(), OverMaxControlLineLimit(),
  ProcessPub/HeaderPub/RoutedMsgArgs/RoutedHeaderMsgArgs, ClonePubArg(), GetHeader()
- tests/Protocol/ProtocolParserTests.cs: 17 tests via TestProtocolHandler stub

Auth extras from session 06 (committed separately):
- Auth/TpmKeyProvider.cs, Auth/CertificateIdentityProvider/, Auth/CertificateStore/

Internal utilities & data structures (session 06 overflow):
- Internal/AccessTimeService.cs, ElasticPointer.cs, SystemMemory.cs, ProcessStatsProvider.cs
- Internal/DataStructures/GenericSublist.cs, HashWheel.cs
- Internal/DataStructures/SubjectTree.cs, SubjectTreeNode.cs, SubjectTreeParts.cs

All 461 tests pass (460 unit + 1 integration). DB updated for features 2588-2592 and tests 2598-2614.

docs/plans/phases/phase6sessions/session-22.md

@@ -0,0 +1,51 @@
+# Session 22: MQTT
+
+## Summary
+
+MQTT 3.1.1/5.0 protocol adapter — allows MQTT clients to connect to NATS and interact with JetStream for persistence.
+
+## Scope
+
+| Go File | Features | Feature IDs | Go LOC |
+|---------|----------|-------------|--------|
+| server/mqtt.go | 153 | 2252–2404 | 4,758 |
+| **Total** | **153** | | **4,758** |
+
+## .NET Classes
+
+- `MqttHandler` — MQTT protocol handler (35 features)
+- `MqttAccountSessionManager` — per-account MQTT session tracking (26 features)
+- `MqttSession` — individual MQTT session state (15 features)
+- `MqttJetStreamAdapter` — bridges MQTT to JetStream (22 features)
+- `MqttReader` — MQTT packet reader (8 features)
+- `MqttWriter` — MQTT packet writer (5 features)
+- Various MQTT reload options
+
+## Test Files
+
+| Test File | Tests | Test IDs |
+|-----------|-------|----------|
+| server/mqtt_test.go | 159 | 2170–2328 |
+| server/mqtt_ex_test_test.go | 2 | 2168–2169 |
+| server/mqtt_ex_bench_test.go | 1 | 2167 |
+| **Total** | **162** | |
+
+## Dependencies
+
+- Session 01 (Foundation Types)
+- Session 08 (Client Connection)
+- Session 09 (Server Core Part 1)
+- Session 11 (Accounts)
+- Session 17 (Store Interfaces)
+- Session 19 (JetStream Core)
+
+## .NET Target Location
+
+- `dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/`
+
+## Notes
+
+- MQTT is a self-contained protocol layer — could potentially be a separate assembly
+- 159 MQTT tests cover connection, subscribe, publish, QoS levels, sessions, retained messages
+- MQTT ↔ JetStream bridging is the most complex part
+- Consider using `System.IO.Pipelines` for MQTT packet parsing