Add MusicBridge protocol documentation and implement system volume control

- Created PROTOCOL.md to outline the MusicBridge protocol v1, detailing setup, TLS, pairing, state management, and command structure.
- Implemented SystemVolume class for managing system audio levels, including methods for setting, reading, changing, and muting volume.
- Added unit tests for the MusicBridge agent, covering pairing, identity storage, command validation, and media state management.
- Included tests for real-time media reading from Windows and ensured proper handling of artwork caching and retrieval.

MusicBridge.Agent.csproj

@@ -5,6 +5,18 @@
     <TargetFramework>net10.0-windows10.0.19041.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
+    <UseWindowsForms>true</UseWindowsForms>
   </PropertyGroup>
 
-</Project>
+  <ItemGroup>
+    <FrameworkReference Include="Microsoft.AspNetCore.App" />
+    <Compile Remove="tests/**/*.cs" />
+    <InternalsVisibleTo Include="MusicBridge.Agent.Tests" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Makaretu.Dns.Multicast" Version="0.27.0" />
+    <PackageReference Include="QRCoder" Version="1.8.0" />
+  </ItemGroup>
+
+</Project>