Content Scene
data class ContentScene(val primary: ResolvedContent?, val local: ResolvedContent?, val remotes: List<ResolvedContent>)
Full content resolution for a render: per-owner resolved content plus the chosen primary. Callers render a content tile per sharer (design: per-peer/per-participant content) from local + remotes.
all is the flat list of every active sharer (remotes then local, mirroring web's ContentScene.all ordering) so the stream-keyed stage can emit one content tile per sharer in a stable order.