docs: clarify Windows build shell preflight

The MVP test guide already asks the tester to verify that the MSVC tools are
available, but the full Windows client build depends on native C tooling through
its dependency stack. Make the expected shell explicit so a missing `cl`, `lib`,
or `link` failure points directly to Visual Studio Build Tools setup instead of
looking like a Rust or project error.

Test Plan:
- git diff --check

Refs: MVP Windows client build handoff

TESTING.md

@@ -87,6 +87,11 @@ 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.
 
+Use an elevated **x64 Native Tools Command Prompt for VS** or **Developer
+PowerShell for VS** for the Windows build and client run. If the commands below
+cannot find `cl`, `lib`, or `link`, install or repair Visual Studio Build Tools
+with the C++ build tools and Windows SDK, then reopen the developer shell.
+
 Quick Windows checks:
 
 ```powershell