fix(client): restore TAP media status on exit

The Windows client already treats TAP route, metric, and MTU changes as scoped
runtime state that is restored when the process exits. TAP media status was the
odd one out: startup marked the adapter connected, but there was no matching
cleanup path to mark it disconnected again.

Add a small TAP media guard that owns an Arc to the opened adapter and marks
media disconnected on drop. The frame pump now receives an Arc<TapAdapter>, so
the guard can run on normal Ctrl-C, startup errors after TAP open, route-check
failures, or frame-pump failures without fighting ownership of the TAP reader
thread. Cleanup errors are logged because Drop cannot return them.

Update the README and MVP test guide so the documented cleanup behavior matches
the client runtime.

The full Windows client cross-check is still blocked on this host before project
code by ring needing x86_64-w64-mingw32-gcc. The TAP and route helper crates
still check for the Windows GNU target.

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

Refs: MVP Windows TAP cleanup

README.md

@@ -218,11 +218,12 @@ The startup status reports whether the relay already has a LAN gateway for the
 room.
 `--virtual-mac` can still override the stored identity for manual testing. On
 Windows it sets the TAP IP interface MTU to the relay-selected MTU, marks the
-TAP media connected, and reports the driver MAC/MTU before forwarding frames,
-along with the TAP interface index/LUID. The client applies a scoped TAP
-interface metric and disables TAP default routes while it runs, periodically
-rechecks that the relay route remains pinned, then restores the previous route
-policy on exit. Startup prints a warning when TAP default routes were enabled
+TAP media connected for the scoped client run, and reports the driver MAC/MTU
+before forwarding frames, along with the TAP interface index/LUID. The client
+applies a scoped TAP interface metric and disables TAP default routes while it
+runs, periodically rechecks that the relay route remains pinned, then restores
+the previous route policy and TAP media status on exit. Startup prints a warning
+when TAP default routes were enabled
 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