feat(client): list TAP adapters before connecting

Add a --list-tap-adapters mode to the Windows client. The command prints the
TAP-Windows6 adapter instance ids and exits without requiring relay, room, or
certificate arguments.

This makes the manual MVP test smoother on machines with multiple TAP adapters:
the operator can ask the binary for the exact ids, then rerun with
--tap-instance-id instead of relying on a separate PowerShell query or waiting
for the ambiguous-adapter startup error.

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

@@ -227,7 +227,8 @@ 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.
+InterfaceGuid. `--list-tap-adapters` prints the TAP adapter ids and exits
+without connecting.
 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