Replace WebTransport-only design with three-transport model:
- Browser↔server: WebSocket (works today, no infrastructure changes)
- Browser↔browser: WebRTC data channels (true P2P, NAT traversal)
- Server↔server: HTTP federation (existing sx-pub)
Home nodes relay WebRTC signaling. Reliable channels for chat/components,
unreliable channels for game state/cursor positions. Transport-agnostic
protocol layer — upgrade to WebTransport later with zero SX changes.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>