deferred_renderer_join joined every connect with is_active=true, so QBZ
grabbed playback from whatever peer was rendering the instant it came
online, and the self-state echo from the post-join AskForRendererState
then reset the cursor to the queue head. Join as an available renderer
(is_active=false) on a fresh connect; only a post-drop RECONNECTION
rejoins as active, so a network blip mid-render does not lose the render.
Taking over is now explicit (device-picker Play here, or the phone
selecting QBZ, both arriving as SET_ACTIVE) - matching the official
clients.
cc
vicrodh
ab44debb75