TASK-033-003: Delete unused ProtocolCoreGraphBuilder.cs

- Delete src/TurboHttp/Streams/ProtocolCoreGraphBuilder.cs (now dead code after TASK-033-002)
- Verified zero remaining references via grep
- Build and test suite pass with zero errors

Author: st0o0

.maggus/features/feature_033.md

@@ -96,10 +96,10 @@ This feature inverts the topology: group by endpoint once at the top level, then
 **Model:** haiku
 
 **Acceptance Criteria:**
-- [ ] `src/TurboHttp/Streams/ProtocolCoreGraphBuilder.cs` is deleted
-- [ ] No remaining `using` or reference to `ProtocolCoreGraphBuilder` anywhere in `src/` (verified via grep)
-- [ ] `dotnet build --configuration Release ./src/TurboHttp.sln` passes with zero errors after deletion
-- [ ] `dotnet test ./src/TurboHttp.sln` passes
+- [x] `src/TurboHttp/Streams/ProtocolCoreGraphBuilder.cs` is deleted
+- [x] No remaining `using` or reference to `ProtocolCoreGraphBuilder` anywhere in `src/` (verified via grep)
+- [x] `dotnet build --configuration Release ./src/TurboHttp.sln` passes with zero errors after deletion
+- [x] `dotnet test ./src/TurboHttp.sln` passes
 
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