feat(relay): add Ethernet forwarding decisions

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

crates/lanparty-relay/src/lib.rs

@@ -8,7 +8,9 @@ use std::collections::HashMap;
 use lanparty_ctrl::{
     ControlError, EndpointHello, PeerInfo, Reject, RejectReason, Role, RoomCode, ServerWelcome,
 };
-use lanparty_proto::{MacAddr, recommended_tap_mtu};
+use lanparty_obs::{DropReason, FrameAction};
+use lanparty_proto::{EthernetFrame, MacAddr, recommended_tap_mtu};
+use thiserror::Error;
 
 pub const DEFAULT_MAX_CLIENTS_PER_ROOM: usize = 16;
 
@@ -65,6 +67,56 @@ impl RoomSnapshot {
     }
 }
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ForwardingDecision {
+    targets: Vec<u32>,
+    action: FrameAction,
+    drop_reason: Option<DropReason>,
+}
+
+impl ForwardingDecision {
+    #[must_use]
+    fn forwarded(targets: Vec<u32>) -> Self {
+        Self {
+            targets,
+            action: FrameAction::Forwarded,
+            drop_reason: None,
+        }
+    }
+
+    #[must_use]
+    fn dropped(drop_reason: DropReason) -> Self {
+        Self {
+            targets: Vec::new(),
+            action: FrameAction::Dropped,
+            drop_reason: Some(drop_reason),
+        }
+    }
+
+    #[must_use]
+    pub fn targets(&self) -> &[u32] {
+        &self.targets
+    }
+
+    #[must_use]
+    pub const fn action(&self) -> FrameAction {
+        self.action
+    }
+
+    #[must_use]
+    pub const fn drop_reason(&self) -> Option<DropReason> {
+        self.drop_reason
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Error)]
+pub enum ForwardingError {
+    #[error("room {0} does not exist")]
+    UnknownRoom(RoomCode),
+    #[error("ingress peer {peer_id} is not present in room {room}")]
+    UnknownIngressPeer { room: RoomCode, peer_id: u32 },
+}
+
 #[derive(Debug, Clone)]
 pub struct RoomRegistry {
     rooms: HashMap<RoomCode, Room>,
@@ -125,6 +177,18 @@ impl RoomRegistry {
         self.rooms.get(room).map(Room::snapshot)
     }
 
+    pub fn forward_ethernet(
+        &self,
+        room: &RoomCode,
+        ingress_peer_id: u32,
+        frame_bytes: &[u8],
+    ) -> Result<ForwardingDecision, ForwardingError> {
+        self.rooms
+            .get(room)
+            .ok_or_else(|| ForwardingError::UnknownRoom(room.clone()))?
+            .forward_ethernet(room, ingress_peer_id, frame_bytes)
+    }
+
     fn allocate_room_id(&mut self) -> Result<u64, Reject> {
         let room_id = self.next_room_id;
         self.next_room_id = self
@@ -260,6 +324,78 @@ impl Room {
             clients,
         }
     }
+
+    fn forward_ethernet(
+        &self,
+        room_code: &RoomCode,
+        ingress_peer_id: u32,
+        frame_bytes: &[u8],
+    ) -> Result<ForwardingDecision, ForwardingError> {
+        let ingress =
+            self.peer(ingress_peer_id)
+                .ok_or_else(|| ForwardingError::UnknownIngressPeer {
+                    room: room_code.clone(),
+                    peer_id: ingress_peer_id,
+                })?;
+        let frame = match EthernetFrame::parse(frame_bytes) {
+            Ok(frame) => frame,
+            Err(_) => return Ok(ForwardingDecision::dropped(DropReason::Malformed)),
+        };
+
+        if ingress.role() == Role::Client {
+            let expected_source = ingress.mac().expect("client peers have MAC addresses");
+            if frame.source() != expected_source {
+                return Ok(ForwardingDecision::dropped(
+                    DropReason::UnauthorizedSourceMac,
+                ));
+            }
+        }
+
+        let targets = if frame.destination().is_multicast() {
+            self.all_peer_ids_except(ingress_peer_id)
+        } else if let Some(client_peer_id) = self.clients_by_mac.get(&frame.destination()) {
+            if *client_peer_id == ingress_peer_id {
+                Vec::new()
+            } else {
+                vec![*client_peer_id]
+            }
+        } else {
+            self.all_peer_ids_except(ingress_peer_id)
+        };
+
+        if targets.is_empty() {
+            return Ok(ForwardingDecision::dropped(DropReason::UnknownDestination));
+        }
+
+        Ok(ForwardingDecision::forwarded(targets))
+    }
+
+    fn peer(&self, peer_id: u32) -> Option<&PeerInfo> {
+        self.gateway
+            .as_ref()
+            .filter(|peer| peer.peer_id() == peer_id)
+            .or_else(|| self.clients.get(&peer_id))
+    }
+
+    fn all_peer_ids_except(&self, ingress_peer_id: u32) -> Vec<u32> {
+        let mut peer_ids =
+            Vec::with_capacity(self.clients.len() + usize::from(self.gateway.is_some()));
+
+        if let Some(gateway) = &self.gateway
+            && gateway.peer_id() != ingress_peer_id
+        {
+            peer_ids.push(gateway.peer_id());
+        }
+
+        peer_ids.extend(
+            self.clients
+                .keys()
+                .copied()
+                .filter(|peer_id| *peer_id != ingress_peer_id),
+        );
+        peer_ids.sort_unstable();
+        peer_ids
+    }
 }
 
 fn reject_control_error(error: ControlError) -> Reject {
@@ -299,6 +435,14 @@ mod tests {
         EndpointHello::gateway(room(), 1400).unwrap()
     }
 
+    fn ethernet(destination: MacAddr, source: MacAddr) -> Vec<u8> {
+        let mut frame = Vec::new();
+        frame.extend_from_slice(&destination.octets());
+        frame.extend_from_slice(&source.octets());
+        frame.extend_from_slice(&0x0800_u16.to_be_bytes());
+        frame
+    }
+
     #[test]
     fn accepts_gateway_and_client_into_room() {
         let mut registry = RoomRegistry::default();
@@ -371,4 +515,104 @@ mod tests {
 
         assert_eq!(gateway.welcome().effective_tap_mtu(), 972);
     }
+
+    #[test]
+    fn forwards_unknown_client_unicast_to_gateway() {
+        let mut registry = RoomRegistry::default();
+        let gateway = registry.join(gateway_hello()).unwrap();
+        let client = registry.join(client_hello(1)).unwrap();
+        let frame = ethernet(MacAddr::new([0x00, 1, 2, 3, 4, 5]), mac(1));
+
+        let decision = registry
+            .forward_ethernet(&room(), client.peer().peer_id(), &frame)
+            .unwrap();
+
+        assert_eq!(decision.action(), FrameAction::Forwarded);
+        assert_eq!(decision.targets(), &[gateway.peer().peer_id()]);
+        assert_eq!(decision.drop_reason(), None);
+    }
+
+    #[test]
+    fn forwards_gateway_unicast_to_matching_client() {
+        let mut registry = RoomRegistry::default();
+        let gateway = registry.join(gateway_hello()).unwrap();
+        let client_one = registry.join(client_hello(1)).unwrap();
+        let client_two = registry.join(client_hello(2)).unwrap();
+        let frame = ethernet(mac(2), MacAddr::new([0x00, 1, 2, 3, 4, 5]));
+
+        let decision = registry
+            .forward_ethernet(&room(), gateway.peer().peer_id(), &frame)
+            .unwrap();
+
+        assert_eq!(decision.action(), FrameAction::Forwarded);
+        assert_eq!(decision.targets(), &[client_two.peer().peer_id()]);
+        assert!(!decision.targets().contains(&client_one.peer().peer_id()));
+    }
+
+    #[test]
+    fn floods_broadcast_without_reflecting_ingress() {
+        let mut registry = RoomRegistry::default();
+        let gateway = registry.join(gateway_hello()).unwrap();
+        let client_one = registry.join(client_hello(1)).unwrap();
+        let client_two = registry.join(client_hello(2)).unwrap();
+        let frame = ethernet(MacAddr::BROADCAST, mac(1));
+
+        let decision = registry
+            .forward_ethernet(&room(), client_one.peer().peer_id(), &frame)
+            .unwrap();
+
+        assert_eq!(decision.action(), FrameAction::Forwarded);
+        assert_eq!(
+            decision.targets(),
+            &[gateway.peer().peer_id(), client_two.peer().peer_id()]
+        );
+    }
+
+    #[test]
+    fn drops_client_frames_with_forged_source_mac() {
+        let mut registry = RoomRegistry::default();
+        let client = registry.join(client_hello(1)).unwrap();
+        let frame = ethernet(MacAddr::BROADCAST, mac(2));
+
+        let decision = registry
+            .forward_ethernet(&room(), client.peer().peer_id(), &frame)
+            .unwrap();
+
+        assert_eq!(decision.action(), FrameAction::Dropped);
+        assert_eq!(
+            decision.drop_reason(),
+            Some(DropReason::UnauthorizedSourceMac)
+        );
+        assert!(decision.targets().is_empty());
+    }
+
+    #[test]
+    fn drops_malformed_frames() {
+        let mut registry = RoomRegistry::default();
+        let client = registry.join(client_hello(1)).unwrap();
+
+        let decision = registry
+            .forward_ethernet(&room(), client.peer().peer_id(), &[0; 4])
+            .unwrap();
+
+        assert_eq!(decision.action(), FrameAction::Dropped);
+        assert_eq!(decision.drop_reason(), Some(DropReason::Malformed));
+    }
+
+    #[test]
+    fn reports_unknown_ingress_peer() {
+        let mut registry = RoomRegistry::default();
+        registry.join(client_hello(1)).unwrap();
+        let frame = ethernet(MacAddr::BROADCAST, mac(1));
+
+        let error = registry.forward_ethernet(&room(), 99, &frame).unwrap_err();
+
+        assert_eq!(
+            error,
+            ForwardingError::UnknownIngressPeer {
+                room: room(),
+                peer_id: 99,
+            }
+        );
+    }
 }