feat(client): require explicit TAP selection when ambiguous

Add a Windows client --tap-instance-id option for selecting a specific
TAP-Windows6 adapter by NetCfgInstanceId / InterfaceGuid. The client still opens
the only installed TAP adapter automatically, but now refuses to choose an
arbitrary adapter when multiple TAP-Windows6 adapters are present.

This keeps the MVP test run from silently configuring and opening the wrong TAP
adapter on machines that already have VPN or test TAP devices installed. The
error lists available adapter instance ids so the operator can rerun with the
intended value.

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

Refs: PLAN.md Windows client TAP adapter responsibility

README.md

@@ -224,6 +224,10 @@ before the scoped protection was applied. Startup still fails before bridging
 if the driver-reported MAC does not match the tunnel identity, because an
 already-initialized Windows TAP adapter may need to be disabled/enabled or
 reinstalled before it reloads the configured `NetworkAddress`.
+If exactly one TAP-Windows6 adapter is installed, the client opens it
+automatically. If multiple TAP-Windows6 adapters are installed, startup fails
+until `--tap-instance-id` selects the intended adapter by NetCfgInstanceId /
+InterfaceGuid.
 It prints and reports client diagnostics snapshots with relay reachability,
 LAN-gateway presence, route-pinning, QUIC datagram budget, relay RTT, TAP
 status/IP, broadcast frame flow, frame/datagram counters, and drops. The