The MVP test guide tells operators that --iface is accepted as a shorter alias
for --interface, but Clap hid the alias from lanparty-gateway --help. Make the
alias visible so the binary's self-documentation matches the handoff guide.
This does not change parsing behavior; --iface was already accepted and covered
by the existing gateway parser test.
Test Plan:
- cargo fmt --check
- cargo test -p lanparty-gateway accepts_iface_alias_for_gateway_interface
- cargo run -p lanparty-gateway -- --help
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
- git diff --cached --check
Refs: MVP gateway test handoff
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