Add a tested relay room layer before introducing QUIC socket handling. The relay
now has a focused place to enforce room membership rules instead of mixing those
rules into the future networking loop.
RoomRegistry accepts validated endpoint hellos, assigns room and peer IDs,
returns server welcome data, limits clients per room, permits only one gateway,
rejects duplicate client MACs, and keeps the room TAP MTU stable once the first
peer joins. A later peer must support the existing room MTU rather than silently
shrinking it after an earlier client may already have configured its TAP adapter.
The networking pieces still need to call this layer from the reliable control
stream and use the resulting peer metadata for datagram forwarding.
Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
Refs: PLAN.md relay responsibilities and MAC identity
ddidderr
