docs: add Windows client preflight checks

The manual MVP proof depends on a real Windows build environment and an
installed TAP-Windows6 adapter. Make the test guide catch those prerequisites
before the operator starts the relay/gateway/client flow.

Add `where.exe link` alongside the existing MSVC tool checks, and add a short
post-build smoke check for `--help` and `--list-tap-adapters`. This keeps the
handoff focused while making missing linker or TAP setup failures show up before
any room is joined.

Test Plan:
- cargo fmt --check
- git diff --check
- git diff --cached --check

Refs: MVP Windows E2E handoff

TESTING.md

@@ -93,6 +93,7 @@ Quick Windows checks:
 rustc -vV
 where.exe cl
 where.exe lib
+where.exe link
 ```
 
 `rustc -vV` should report a `host` containing `x86_64-pc-windows-msvc`.
@@ -111,6 +112,16 @@ On Windows, in an Administrator terminal:
 cargo build --release -p lanparty-client-win
 ```
 
+Then confirm the binary starts and can see TAP-Windows6:
+
+```powershell
+.\target\release\lanparty-client-win.exe --help
+.\target\release\lanparty-client-win.exe --list-tap-adapters
+```
+
+The adapter list should show at least one TAP-Windows6 adapter before the full
+tunnel run. If it does not, fix the TAP install first.
+
 The binaries used below are:
 
 ```text