Implements WebSocketStreamAdapter — a Stream subclass that wraps
System.Net.WebSockets.WebSocket for use by LeafConnection. Handles
message framing (per-message receive/send), tracks BytesRead/BytesWritten
and MessagesRead/MessagesWritten counters, and exposes IsConnected. Ten
NSubstitute-based unit tests cover all capability flags, delegation, and
telemetry (10/10 pass).
