Client-originated Ethernet frames now pass through a per-peer byte token bucket
after source and safety validation and before destination-specific forwarding.
The fixed MVP default allows a 4 MiB burst with a 2 MiB/s refill, giving normal
LAN-game traffic headroom while preventing one client from filling the relay
path indefinitely.
The existing frame burst buckets now share the same `TokenBucket` type with a
unit cost of one frame. Keeping this state in `Room` preserves forwarding-policy
ownership and lets tests drive explicit instants.
This completes the relay-side abuse limit list from PLAN.md. Configurable rate
limits remain future work.
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
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