Field report from the first live test of the fMP4 ingest: a MistServer container still running the legacy MKVExec process config POSTed MKV to /live on a 4s restart loop, and each attempt died as an instant cryptic qtdemux failure (a pile of ~200-byte truncated debug recordings). Meanwhile the pull ingest dialed the old default port 18080 while Mist listened on 28080, so it never connected at all. - buildMP4IngestPipeline now peeks the stream and rejects the EBML magic with an error that names the actual problem (legacy MKVExec config) instead of letting qtdemux die confusingly. - mist-http-port default 18080 -> 28080, matching docker/mistserver.json (the generated dev config derives Mist's listener from the same flag, so both worlds stay consistent). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>