Per Rames: add the engine audio mixer (audioMixer.ts:350 -> ffmpeg -ss) to the
media-start-only readers — it's the live path that makes a lone data-playback-start
a shipped output bug (trimmed picture over untrimmed audio). 'Set one' isn't enough;
give the element-kind rule: <video>/<audio> use data-media-start, nested composition
uses data-playback-start (Studio writes it; it's the child-timeline offset). Also fix
editing-existing-videos.mdx, which offered both names as interchangeable for front
trim of a clip — the exact kind where they aren't.
ukimsanov
c49fea2e0c