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

TESTING.md

@@ -104,8 +104,7 @@ If the Windows machine has multiple TAP-Windows6 adapters, select the intended
 one explicitly:
 
 ```powershell
-Get-NetAdapter | Where-Object InterfaceDescription -Like "*TAP*" |
-  Select-Object Name, InterfaceGuid, InterfaceDescription
+.\target\release\lanparty-client-win.exe --list-tap-adapters
 
 .\target\release\lanparty-client-win.exe `
   --relay relay.example.net:8443 `