feat(client): expose tunnel traffic counters

Client relay I/O now shares atomic tunnel counters across cloned
ClientRelayIo handles and the owning ClientSession. The counters cover
successful Ethernet frame rx/tx, relay datagram rx/tx, and dropped or
malformed frames so client diagnostics have the traffic totals called
out in PLAN.md.

The counters live in client-core because that crate owns relay datagram
classification and Ethernet payload validation. The Windows TAP runner can
later sample this snapshot without duplicating protocol decisions.

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

Refs: PLAN.md

README.md

@@ -48,6 +48,7 @@ Platform-neutral remote client relay session:
 - client hello with room, virtual MAC, and datagram budget
 - welcome/reject handling with assigned peer id and effective TAP MTU
 - Ethernet frame send/receive helpers over QUIC DATAGRAM
+- client tunnel statistics for frame/datagram rx/tx and drops
 
 ### `lanparty-client-route`