The gateway previously joined the relay room before opening the Linux
AF_PACKET socket. If the operator passed the wrong interface, a wireless
interface, or an interface that could not be opened, the relay could briefly
advertise a gateway that was never able to bridge LAN traffic.
Parse CLI args first, then keep platform handling in a small cfg-gated run
function. On Linux, build the config, open the AF_PACKET socket, and only then
join the relay as the room gateway. On non-Linux targets, fail before reading
relay certificate files, resolving relay names, or opening network connections.
Update the README and MVP test guide so the documented gateway startup order
matches the binary output.
Test Plan:
- cargo fmt --check
- cargo test -p lanparty-gateway
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
Refs: MVP gateway startup hardening
ddidderr
47f66b7d04
