feat(client): report TAP interface identity

Resolve the opened TAP adapter's NetCfgInstanceId to its Windows interface
index and LUID during startup, then print those values with the existing TAP
MAC/MTU diagnostics. This makes the interface identity visible before the next
metric-setting slice uses it for route protection.

The lookup failure is treated as startup failure because an opened TAP adapter
that cannot be resolved as a Windows network interface is not a good candidate
for metric or route management.

Verification note: I attempted to check `lanparty-client-win` for
`x86_64-pc-windows-msvc`, but this host still lacks the Windows C headers
needed by `ring`; the build stops at `assert.h` before the binary crate can be
typechecked for Windows.

Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- cargo check -p lanparty-client-route --target x86_64-pc-windows-msvc
- cargo clippy -p lanparty-client-route --target x86_64-pc-windows-msvc --all-targets -- -D warnings
- git diff --check

Refs: PLAN.md

README.md

@@ -137,5 +137,6 @@ and then bridges Ethernet frames between the relay and the first TAP-Windows6
 adapter until shutdown.
 `--virtual-mac` can still override the stored identity for manual testing. On
 Windows it marks the TAP media connected and reports the driver MAC/MTU before
-forwarding frames. Default-route takeover neutralization and automatic TAP
-MAC/MTU configuration are not wired yet.
+forwarding frames, along with the TAP interface index/LUID. Default-route
+takeover neutralization and automatic TAP MAC/MTU configuration are not wired
+yet.

crates/lanparty-client-win/src/main.rs

@@ -185,6 +185,9 @@ fn print_pinned_relay_route(route: &PinnedRelayRoute) {
 fn open_tap_adapter(session: &ClientSession) -> Result<lanparty_client_tap::TapAdapter> {
     let tap = lanparty_client_tap::open_first_adapter()?;
     tap.set_media_connected(true)?;
+    let tap_interface =
+        lanparty_client_route::interface_identity_from_guid(tap.info().instance_id())
+            .context("failed to resolve TAP interface identity")?;
     let driver_mac = tap.driver_mac()?;
     let driver_mtu = tap.driver_mtu()?;
 
@@ -198,6 +201,11 @@ fn open_tap_adapter(session: &ClientSession) -> Result<lanparty_client_tap::TapA
         driver_mtu,
         session.welcome().effective_tap_mtu()
     );
+    println!(
+        "TAP interface index {} LUID {}",
+        tap_interface.index(),
+        tap_interface.luid()
+    );
     if driver_mac != session.config().virtual_mac() {
         eprintln!(
             "TAP driver MAC {} does not match tunnel identity {}; MAC configuration is not wired yet",