docs: spell out Windows build prerequisites

The MVP test guide asked for a Windows release build but did not make the MSVC
native-toolchain dependency explicit. The client dependency stack includes
native code, so a fresh Windows test machine needs the Rust MSVC host and C++
Build Tools available before cargo can build lanparty-client-win.

Add a short prerequisite section with quick checks for rustc host, cl.exe, and
lib.exe. This gives the manual Windows tester a faster explanation if the build
environment is missing Visual Studio Build Tools.

Test Plan:
- cargo test -p lanparty-client-win --help >/dev/null
- git diff --check
- git diff --cached --check

Refs: MVP Windows test handoff

TESTING.md

@@ -38,6 +38,23 @@ LAN test host:  <lan-host-ip>
 
 Use the same room code everywhere, for example `ROOM1`.
 
+## Build Prerequisites
+
+On Windows, use the Rust MSVC toolchain and install Visual Studio Build Tools
+with the C++ build tools. The dependency stack includes native code, so tools
+such as `cl.exe` and `lib.exe` must be available in the build environment.
+TAP-Windows6 must be installed before running the client.
+
+Quick Windows checks:
+
+```powershell
+rustc -vV
+where.exe cl
+where.exe lib
+```
+
+`rustc -vV` should report a `host` containing `x86_64-pc-windows-msvc`.
+
 ## Build
 
 On the relay or Linux build host: