U cell: receive the pushed UDP RTP stream (udp://@:5600) in the ninth cell and the U view #87

#71 added the ninth multi-view cell and the U button, but they only show a hint. #72 added the receiver (udp:// slots, RtpListener). Wire them together: the ninth cell and the U view listen on port 5600 and show the stream as soon as a camera sends it, with the hint shown until the first frame. Device-checked in #74: Majestic outgoing to udp://<box>:5600 plays in a udp://@:5600 slot.

Device address in the hint

Today Decoder.localIpv4() returns the first site-local IPv4 address once, when the cell is created, and udpHintText() shows only that address.

• All addresses. Show every IPv4 address of every interface that is up, except loopback (127.0.0.0/8) and link-local (169.254.0.0/16), one per line as <address>:5600. Public addresses count too, not only site-local ones. A box with Ethernet and Wi-Fi then shows both, and the user picks the one the camera can reach.
• Keep it current. Refresh the list when the network changes (ConnectivityManager.NetworkCallback: available / lost / link properties changed), so a new DHCP lease or a switch between Ethernet and Wi-Fi shows up without restarting the grid. The same text in the U view.
• No address at all: keep this device:5600.
• Unit test for the text with zero, one and several addresses.