RTSP multicast and other RTSP transports (research: Majestic RTSP has no multicast) #69

Split out of #7, together with #67 (UDP unicast RTSP) and #68 (Majestic outgoing RTP). Everything else about RTSP transports lives here: RTSP-negotiated multicast first of all, and any other transport variants (RTSP over HTTP tunnelling, RTSPS, ...) if they are ever needed.

Feasibility with OpenIPC: Majestic RTSP does not do multicast. Checked on 2026-10-12 on 172.17.32.133 and 172.17.32.126 (Lite SigmaStar infinity6b0, master+012cd47, Server: OpenIPC.org RTSP Server/0.1):

The SDP has c=IN IP4 0.0.0.0 and no multicast address. The RTSP server takes only client_port; a multicast request is either rejected or downgraded to unicast.

Conclusion: for OpenIPC cameras, multicast is only available through Majestic outgoing (plain RTP to udp://<group>:port), #68. RTSP multicast in the decoder would only help with third-party servers and NVRs (live555-based, some Hikvision/Dahua, VLC/ffserver-style servers).

If it is done anyway

• SETUP with RTP/AVP;multicast, parse destination, port, ttl; fall back to UDP unicast / TCP when the reply is 400/461 or comes back as unicast.
• Join the group with MulticastSocket (+ MulticastLock on Wi‑Fi), reuse the receive path of Receive plain RTP from Majestic outgoing over UDP (unicast and multicast) #68.
• Needs a server that supports it for testing; none in the lab right now.
• Majestic side: adding multicast to the RTSP server would be a separate issue in OpenIPC/majestic.

Deferred until there is a concrete server that needs it.