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

crates/lanparty-client-core/Cargo.toml

@@ -8,6 +8,7 @@ anyhow.workspace = true
 bytes.workspace = true
 getrandom.workspace = true
 lanparty-ctrl = { path = "../lanparty-ctrl" }
+lanparty-obs = { path = "../lanparty-obs" }
 lanparty-proto = { path = "../lanparty-proto" }
 quinn.workspace = true
 rustls.workspace = true