feat(gateway): log bridged Ethernet frames

The gateway bridge now emits structured frame log lines for successful LAN to
relay and relay to LAN forwarding. Logs include the physical interface,
direction, peer id when one is known, MACs, ethertype or length field, frame
length, action, and drop reason.

This uses the shared `lanparty-obs` frame vocabulary instead of adding a second
ad hoc diagnostics model to the gateway. The log line stays local to the
gateway because the relay still owns its own room/target-specific formatting.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-gateway
- cargo clippy -p lanparty-gateway --all-targets -- -D warnings
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check

Refs: PLAN.md

crates/lanparty-gateway/Cargo.toml

@@ -8,6 +8,7 @@ anyhow.workspace = true
 bytes.workspace = true
 clap.workspace = true
 lanparty-ctrl = { path = "../lanparty-ctrl" }
+lanparty-obs = { path = "../lanparty-obs" }
 lanparty-proto = { path = "../lanparty-proto" }
 libc.workspace = true
 quinn.workspace = true