mockTransportEnabled

Whether the virtual demo transports (m mock / r replay) may be offered and bound right now.

Reactive rather than a one-shot check because it can flip at runtime: on Android the Demo Mode gesture (five taps on the Settings app-version row) unlocks it mid-session, and the Connections device list has to notice. Every consumer must read this one flow — the device-list visibility path and the isAddressValid admission path have to agree, or the demo entry appears and then refuses to connect.