PLAN.md keeps MVP forwarding to one Ethernet frame per QUIC datagram with no
fragmentation. The relay already skipped targets whose negotiated datagram
budget could not carry an encoded forwarded frame, but that skip was silent.
Retain the no-fragmentation behavior and log the skipped egress target with the
room, ingress peer, target peer, encoded length, and target datagram budget.
Store the peer id in relay sessions so the diagnostic can identify the skipped
target directly.
Document the egress budget-skip diagnostic in the relay README section.
Test Plan:
- cargo fmt --check
- cargo test -p lanparty-relay
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
Refs: PLAN.md
ddidderr
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