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-16.md

@@ -0,0 +1,47 @@
+# Session 16: Gateways
+
+## Summary
+
+Gateway connections — inter-cluster message routing. Gateways enable NATS super-clusters where messages flow between independent clusters.
+
+## Scope
+
+| Go File | Features | Feature IDs | Go LOC |
+|---------|----------|-------------|--------|
+| server/gateway.go | 91 | 1263–1353 | 2,816 |
+| **Total** | **91** | | **2,816** |
+
+## .NET Classes
+
+- `GatewayHandler` — gateway connection management
+- `GatewayCfg` — gateway configuration
+- `ServerGateway` — per-server gateway state
+- `GatewayInterestMode` — interest/optimistic mode tracking
+- `GwReplyMapping` — reply-to subject mapping for gateways
+- `ClientConnection` (partial — gateway-specific methods)
+
+## Test Files
+
+| Test File | Tests | Test IDs |
+|-----------|-------|----------|
+| server/gateway_test.go | 88 | 600–687 |
+| **Total** | **88** | |
+
+## Dependencies
+
+- Session 01 (Foundation Types)
+- Session 07 (Protocol Parser)
+- Session 08 (Client Connection)
+- Session 09 (Server Core Part 1)
+- Session 11 (Accounts — for interest propagation)
+- Session 14 (Routes)
+
+## .NET Target Location
+
+- `dotnet/src/ZB.MOM.NatsNet.Server/Gateway/`
+
+## Notes
+
+- Gateway protocol has optimistic and interest-only modes
+- Account-aware interest propagation is complex
+- 88 tests — thorough coverage of gateway scenarios