Fix CI build: add missing ProcessIdentifierHelper compile include to test project (#34)

AudioStream.cs references ProcessIdentifierHelper but it was not included
in the test project's compile list, causing CS0103 on CI.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: PalmarHealer

ControlPad.Tests/ControlPad.Tests.csproj

@@ -20,6 +20,7 @@
 
   <ItemGroup>
     <Compile Include="../ControlPad/AudioStream.cs" Link="Source/AudioStream.cs" />
+    <Compile Include="../ControlPad/System/ProcessIdentifierHelper.cs" Link="Source/ProcessIdentifierHelper.cs" />
     <Compile Include="../ControlPad/ActionType.cs" Link="Source/ActionType.cs" />
     <Compile Include="../ControlPad/ButtonAction.cs" Link="Source/ButtonAction.cs" />
     <Compile Include="../ControlPad/ButtonCategory.cs" Link="Source/ButtonCategory.cs" />