Replace anacrolix/upnp with huin/goupnp + custom NAT-PMP (RFC 6886)
implementation. NAT-PMP is tried first (faster, more compatible with
TP-Link routers), with UPnP-IGD SOAP as fallback. Gateway detection
reads /proc/net/route for accuracy. Includes unit tests with mock
NAT-PMP server and permanent e2e tests (build tag manual).
- Add UPnP discovery and automatic port mapping (like Plex Remote Access)
- Stream server binds to 0.0.0.0 and reports public IP via UPnP
- Fallback chain: UPnP public IP → Tailscale IP → LAN IP
- Clean up port mapping on shutdown
- Bump version to 0.3.0-dev