WebRTC SFU for game spectating

Design a WebRTC SFU (Selective Forwarding Unit) cluster for real-time multiplayer game spectating / co-watch where a host's gameplay video + audio is relayed to up to 100K live spectators per room with <500ms latency, and a subset (party of 8) are in a low-latency two-way voice/video chat over the same session. You can't mesh 100K peers and you can't transcode per-viewer at that fan-out, so you need an SFU topology that forwards selectively, scales horizontally across many SFU nodes, and degrades gracefully on bad networks.