Add a production-faithful test harness for SignalR binary protocol, introducing SlabTransportWriter to simulate Kestrel's slab allocator and always force the BufferWriterOutput owned-buffer path. Add large-payload round-trip tests (including non-ASCII cases) to catch position drift and data corruption bugs. Enhance protocol tests to validate multi-segment output and byte-for-byte correctness. All protocol round-trips now exercise the multi-segment, non-array-backed buffer path.
