Both sites default to site_id=0; gateway must rewrite site_id in
announcements and protocol messages crossing the boundary. site_id=0
on a cross-site link is a gateway protocol error.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Covers all layers: TCP → transport frame → message type dispatcher →
payload schemas. Documents frame format (6-byte header), all message
types, STREAM_OPEN/CLOSE lifecycle, codec/pixel_format/origin tables,
stream events, and discovery announcement wire format.
Also fixes stale channel_id reference in audio section of architecture.md.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
