feat(relay): classify safety rejects as filtered

PLAN.md describes relay frame logs with separate forwarded, dropped, filtered,
and rate-limited actions. The relay had the shared `Filtered` action in its log
vocabulary, but safety-policy rejects were still reported as generic drops.

Classify forged client source MACs and L2 safety-filter matches as filtered
forwarding decisions. Malformed frames and unknown destinations remain drops,
while rate limits continue to use the rate-limited action.

Document the distinction 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

README.md

@@ -127,6 +127,8 @@ self-signed development certificate; `--dev-cert-der-out` writes that
 certificate so the gateway and client can pin it in development. Production
 certificate handling remains future work. Ethernet forwarding decisions are
 logged with room, peer, MAC, ethertype, action, drop reason, and target count.
+Safety-policy rejects use the `filtered` action so they are distinguishable
+from malformed/unknown-destination drops and rate limits.
 Unknown unicast from a client is forwarded only to the gateway port; unknown
 unicast from the gateway is dropped instead of flooded to every remote client.
 When a peer joins or leaves, the relay sends a reliable lifecycle control event