fix(client): clear TAP media before relay connect

If the Windows client is killed hard, the TAP adapter can be left in a connected
media state. A retry should not resolve or connect to the relay while stale TAP
state might still influence Windows routing.

Select and open the intended TAP adapter before relay endpoint resolution, force
its media state to disconnected, then proceed with the existing relay connect,
route pin, TAP route protection, and bridge startup flow. This also makes
missing or ambiguous TAP adapters fail before the client joins the relay room.
The README and MVP test guide now show the new startup line and the early TAP
preflight troubleshooting checks.

Test Plan:
All cargo commands used these environment variables:
RUSTUP_HOME=/tmp/softlan-vpn-rustup
CARGO_HOME=/tmp/softlan-vpn-cargo

- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- cargo check -p lanparty-client-tap --tests
  --target x86_64-pc-windows-gnu
- cargo check -p lanparty-client-tap --tests
  --target x86_64-pc-windows-msvc
- cargo check -p lanparty-client-route --tests
  --target x86_64-pc-windows-gnu
- cargo check -p lanparty-client-route --tests
  --target x86_64-pc-windows-msvc
- git diff --check

Known limitation: full lanparty-client-win Windows cross-check is still blocked
on this Linux host by the external ring toolchain setup. The default GNU target
lacks x86_64-w64-mingw32-gcc, the default MSVC target lacks lib.exe, and the
LLVM MSVC attempt gets as far as ring C compilation but lacks Windows CRT
headers such as assert.h.

Refs: PLAN.md route-protection startup requirement

TESTING.md

@@ -118,6 +118,7 @@ one explicitly:
 Expected client output:
 
 ```text
+prepared TAP adapter ... media disconnected before relay connect
 lanparty-client-win connected as peer ...
 relay route pinned before TAP ...
 relay route verified after TAP activation ...
@@ -218,6 +219,11 @@ drop_reason=ControlPlaneEtherType
 If the client says `Waiting for LAN gateway`, check that the gateway uses the
 same room code and is connected to the same relay.
 
+If startup fails before the relay connection while preparing the TAP adapter,
+check that the terminal is elevated, TAP-Windows6 is installed, and
+`--tap-instance-id` selects the intended adapter when more than one TAP adapter
+exists.
+
 If the client says `Waiting for TAP IP`, DHCP is not making the full round trip.
 Check relay/gateway frame logs for broadcast traffic and check that the gateway
 is on wired Ethernet.