Add socket-free relay forwarding logic for Ethernet datagrams. The future QUIC
relay loop can now ask the room registry which peer IDs should receive a frame
instead of embedding switching policy in network IO code.
Forwarding validates that the ingress peer belongs to the room, drops malformed
Ethernet frames, rejects client frames whose source MAC does not match the MAC
announced during admission, never reflects frames back to ingress, routes known
client unicasts directly, and floods broadcast/multicast or unknown unicast
frames to the other room peers. The decision reports shared observability action
and drop-reason values so the networking layer can log consistently.
This still does not send bytes over QUIC; it only defines the room-local switch
decision that the datagram loop will use.
Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
Refs: PLAN.md Switching model
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
