fix(gateway): account malformed LAN captures

The Linux AF_PACKET read helper discarded malformed or runt LAN captures before
the bridge loop saw them. That made those frames invisible to gateway drop
counters and frame logs, which is not great for the phase-one heavy diagnostics
called for in the plan.

Return raw inbound capture bytes from the read helper and let the bridge loop
make the drop decision. Malformed LAN frames are now counted as malformed drops,
logged with the normal gateway frame log shape, and skipped without stopping the
bridge. Valid LAN frames still flow through the existing send path and budget
checks.

Document the accounting behavior in the gateway README section.

Test Plan:
- cargo fmt --check
- git diff --check
- cargo test -p lanparty-gateway
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings

Refs: PLAN.md

README.md

@@ -179,10 +179,12 @@ emits small CAM refresh frames so the physical switch keeps those MACs
 associated with the gateway port. Gateway
 frame logs include direction, peer id when present, MACs, ethertype/length,
 frame length, action, and drop reason. The gateway also tracks frame/datagram
-counters and periodically sends stats snapshots to the relay. Relay lifecycle
-events seed and retire remote-client MACs for CAM refresh even before that
-client sends traffic. On shutdown, the gateway sends a best-effort disconnect
-control message before closing QUIC so the relay can report the intended reason.
+counters and periodically sends stats snapshots to the relay. Malformed or runt
+LAN frames are counted and logged as dropped instead of disappearing before
+accounting. Relay lifecycle events seed and retire remote-client MACs for CAM
+refresh even before that client sends traffic. On shutdown, the gateway sends a
+best-effort disconnect control message before closing QUIC so the relay can
+report the intended reason.
 
 ## Windows Client