feat(client): add relay CLI for Windows binary

lanparty-client-win now has a real command-line surface for the relay-facing
client session. It accepts the relay address, expected TLS server name, pinned
DER relay certificate, room code, virtual TAP MAC, and advertised datagram
budget, then connects through lanparty-client-core as role = client.

The binary reports the assigned peer id, room id, and effective TAP MTU from the
welcome response, then waits for Ctrl-C. TAP adapter binding and Windows route
pinning remain future slices, but the executable now exercises the real relay
control-plane path instead of the starter placeholder.

Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings

Refs: PLAN.md Windows client relay connection

README.md

@@ -93,3 +93,19 @@ The gateway connects to the relay as `role = gateway`, completes the
 control-stream hello/welcome handshake, opens an AF_PACKET socket on the LAN
 interface, and bridges Ethernet frames between the relay and wired LAN until
 shutdown.
+
+## Windows Client
+
+```bash
+cargo run -p lanparty-client-win -- \
+  --relay 203.0.113.10:443 \
+  --server-name lanparty-relay.local \
+  --relay-ca-cert relay-cert.der \
+  --room ROOM1 \
+  --virtual-mac 02:00:00:00:00:51
+```
+
+The Windows client binary currently connects to the relay as `role = client`
+with the configured virtual MAC, completes the control-stream hello/welcome
+handshake, and then waits for shutdown. TAP adapter binding and route pinning
+are not wired yet.