feat(relay): track peer last-seen times

The plan calls for relay switch state to remember when peers were last seen.
Wrap room peers in a small entry that keeps PeerInfo alongside a monotonic
Instant, and expose the timestamp through room snapshots for relay diagnostics.

Joining initializes last_seen. Valid Ethernet frames with the peer's authorized
source refresh the timestamp before safety filters or rate limits run, so
filtered but well-formed traffic still proves the peer is active. Malformed
frames and forged client source MACs do not refresh it.

Room leave now removes the associated timestamp with the peer entry. That keeps
membership, MAC indexes, rate-limit buckets, and diagnostics state aligned.

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

Refs: PLAN.md

README.md

@@ -93,6 +93,7 @@ Public relay binary and relay-owned room state:
 - malformed peer datagram disconnect threshold
 - peer stats control events retained for relay diagnostics
 - graceful disconnect control events propagated as peer-leave reasons
+- per-peer last-seen timestamps in relay room snapshots
 - peer leave cleanup for room membership and MAC indexes
 
 ## Build