Receive plain RTP from Majestic outgoing over UDP (unicast and multicast) #68

Split out of #7.

Majestic can push the stream itself, with no RTSP: outgoing destinations (/api/v1/outgoing.json, web UI "Outgoing") with udp://host:port send plain RTP. This is the usual OpenIPC setup for FPV and for feeding several receivers from one encoder. Add a slot type that receives it.

What the stream looks like (measured on hi3516av300 Majestic):

• Always RTP, never MPEG-TS or a bare Annex-B stream.
• PT 96 = H.264 (RFC 6184), PT 97 = H.265 (RFC 7798); FU-A / FU fragments, ~1200 bytes (rtsp.naluSize).
• SPS/PPS (and VPS) are repeated in band before every IDR, so a receiver can join mid-stream with no SDP.

Scope

• Slot URL form, e.g. udp://@:5600 (unicast, listen on a port) and udp://@239.1.2.3:5600 (multicast group, optional source).
• Receiver: DatagramSocket for unicast, MulticastSocket.joinGroup() on the active interface for multicast. Large SO_RCVBUF.
• Wi‑Fi: hold a WifiManager.MulticastLock while a multicast slot is playing; add CHANGE_WIFI_MULTICAST_STATE to the manifest. Ethernet boxes (AG-216) probably do not need it; verify.
• Codec from PT (96/97), with a per-slot override for non-default PTs. Decoder configured from in-band parameter sets: no output until the first SPS/PPS + IDR.
• Feed packets to the existing depacketizer through onPacket(CHANNEL_VIDEO, ...); no RTSP, so "connected"/watchdog is based on incoming packets only.
• Reorder/loss handling as in RTSP over UDP unicast, with fallback to TCP #67 (shared code).
• Audio: Majestic can add audio to a destination; decide whether to support it here or later (the PT and codec are not described anywhere, so it needs a per-slot setting).
• Several slots on different ports / groups at the same time; two slots on the same port must be rejected.

To verify first

• That Majestic actually sends to a multicast group address (udp://239.x.x.x:port) and with what TTL; not checked yet.

Tests

• Unit: URL parsing, PT → codec.
• Device (methodology of Device testing: v2 and open PRs (report + checklist) #53): unicast and multicast from group A cameras, H.264 and H.265, single view and multi-view; join mid-stream; camera restart while playing.