Peers announce a QUIC datagram budget in their hello, and the relay clamps that
value against the transport's negotiated max before room admission. The relay
used that clamped value for MTU selection, but stored the raw transport budget
in the live peer session. A peer that intentionally advertised a smaller budget
could therefore receive egress datagrams larger than it promised to accept.
Store the post-clamp hello budget in AcceptedPeer and PeerSession instead. That
keeps the existing relay egress skip path tied to the same negotiated size used
for room MTU decisions.
The handshake regression now advertises a budget below the QUIC transport
budget and asserts that the accepted peer records the advertised value. The
README decomposition also calls out the per-peer egress-budget invariant.
Test Plan:
- cargo fmt --check
- cargo test -p lanparty-relay
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
- git diff --cached --check
Refs: PLAN.md No fragmentation for MVP
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