fix(relay): print development cert path

The manual MVP run depends on copying the generated development certificate
from the relay host to both the gateway and Windows client. The relay already
wrote the file, but the startup log only said that the relay was listening.

Print the configured development certificate path after a successful bind/write
and align TESTING.md's expected relay output with that line. This gives the
tester an explicit file path to copy before starting the gateway and client.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-relay
- timeout 2s cargo run -q -p lanparty-relay -- --listen 127.0.0.1:0
  --dev-cert-der-out <tmp-cert>
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings

Refs: MVP relay certificate handoff

TESTING.md

@@ -155,6 +155,7 @@ Expected relay output:
 
 ```text
 lanparty-relay configured for 0.0.0.0:8443/udp ...
+lanparty-relay wrote development certificate to relay-cert.der
 lanparty-relay listening on 0.0.0.0:8443
 ```