Replace the placeholder relay binary with a typed command-line configuration
entry point. This gives the future QUIC server loop the listen endpoint and room
limit configuration it needs without mixing command parsing into networking or
room-state code.
The relay now accepts --listen as either a socket address or an explicit UDP
shorthand such as 443/udp, defaults to 0.0.0.0:443/udp, and validates that the
per-room client limit is positive. The binary currently reports the parsed
configuration and clearly states that the QUIC server loop is not wired yet, so
this commit does not pretend to provide a working relay.
Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- cargo run -p lanparty-relay -- --listen 443/udp
Refs: PLAN.md public relay --listen requirement
Add the framing layer that turns typed control messages into bytes for reliable
QUIC streams. This keeps the codec next to the control schema while leaving the
actual QUIC read/write loops for a later relay/client/gateway slice.
The codec uses a four-byte big-endian length prefix followed by JSON. JSON is a
phase-1 choice for inspectability during manual tunnel bring-up; the explicit
length prefix keeps stream parsing deterministic and the 64 KiB cap prevents a
peer from announcing unbounded control payloads. Decoding validates the message
after deserialization so forged stream bytes cannot bypass constructor checks.
The next networking slice can use complete_control_frame_len to split a stream
buffer and decode_control_frame once a complete frame is available.
Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
Refs: PLAN.md reliable QUIC control stream requirements
ddidderr
