Transport: show the transport actually in use, and Auto as the default #112

1. Show the actual transport. The debug page (cell top line), /stats.json and the corner text show the configured mode of a slot (TCP, UDP, AUTO, MULTICAST). With Auto or Multicast the session may fall back (UDP SETUP failed (…), using TCP, no video over UDP, using TCP, multicast SETUP failed), and the label then says AUTO although the stream runs over TCP.

• RtspSession exposes the transport in use after PLAY (TCP / UDP / multicast; udp:// slots and the U cell: UDP, or multicast for a group).
• /stats.json: transport = in use, transport_mode = configured; the page shows the one in use, e.g. UDP, or TCP (auto) after a fallback.
• Single view: the same in /stats.json single.

2. Auto by default. A slot without a stored transport (new install, new slot, a shared link without transport) uses Auto instead of TCP: UDP when it works, TCP otherwise. Stored choices stay as they are. URL dialog: Auto preselected for an empty slot.

Lab: all slots on Aceline-1 and Aceline-2 switched to Auto by hand (2026-10-13).