The Windows client frame pump needs one side reading TAP frames and sending them
to the relay while another side receives relay datagrams and writes them to TAP.
That should not require the binary to reach into `ClientSession` internals.
Introduce `ClientRelayIo`, a cloneable handle around the accepted QUIC
connection and server welcome. It owns the Ethernet datagram send/receive logic
that previously lived directly on `ClientSession`, while `ClientSession` keeps
convenience forwarding methods for existing callers.
This is an enabling slice only. TAP frame pumping can now hold independent relay
I/O handles without broadening the platform-specific TAP crate.
Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- Windows-target cargo clippy for lanparty-client-tap with -D warnings
- git diff --check
Refs: PLAN.md Windows TAP frame pump
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