A media stream advertisement now carries `media_endpoint`, where the
producer serves it from; the request no longer carries a consumer endpoint.
The producer listens and the consumer connects, the same way as every other
client/server pair on the mesh, so a viewer's host admits nothing inbound
and needs no firewall rule or port forward. Unshipped contract, keys
renumbered in place.
Audio gets its own channel ("audio": reliable, unordered, media expiry) so
one session can carry lossy video under parity and reliable audio together,
which is what lets the producer serve both to one dialled-in consumer.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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