[Realtime] WebSocket transport as a nested Go module

[cssbruno]

Implement the WebSocket presentation-event transport as a nested Go module.

Scope:

• Keep WebSocket dependencies out of the root module graph.
• Expose the same presentation-event fanout contract as the dependency-free transport where practical.

Acceptance criteria:

• Nested module builds and tests independently.
• Root go list -m all does not include WebSocket-only dependencies.
• Docs show opt-in usage.