feat(relay): disconnect peers after malformed datagrams

The relay now tracks malformed datagrams per accepted peer and closes the QUIC
connection after a small threshold. Malformed overlay bytes, datagrams with the
wrong room/peer/type header, and malformed Ethernet payloads all count toward
that threshold.

This implements the malformed-packet disconnect part of PLAN.md without mixing
in broader bandwidth or broadcast rate limiting. Ordinary safety-filter drops
still remain non-fatal; this only targets peers that repeatedly send packets the
relay cannot treat as valid tunnel Ethernet traffic.

The threshold state lives in the relay server loop, while the forwarding helper
returns a small outcome enum so malformed classification stays testable without
running a full QUIC server. The room registry remains responsible for Ethernet
policy decisions such as unauthorized source MACs, jumbo frames, and control
plane filters.

Test Plan:
- cargo fmt --check
- 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

@@ -80,6 +80,7 @@ Public relay binary and relay-owned room state:
 - stable effective room MTU chosen before Ethernet datagrams flow
 - live Ethernet datagram forwarding with no ingress reflection
 - L2 safety filters for jumbo, switch-control, DHCP-server, and IPv6-RA frames
+- malformed peer datagram disconnect threshold
 - peer leave cleanup for room membership and MAC indexes
 
 ## Build