Copy RTP payload bytes when parsing packets so buffered packets do not retain slices into the shared UDP read buffer.

Previously ParseRTPPacket() kept Payload as a subslice of the caller-provided buffer. Once the next ReadFromUDP() reused that buffer, any packet still waiting inside the jitter buffer would silently see corrupted payload data. In practice this could surface as clicks, artifacts, or silent audio on reordered AES67/RTP traffic without obvious decoder errors.

Take an owned copy of the payload on parse so jitter-buffered packets remain stable even when the source read buffer is reused.