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.

Network/PairingQr.cs

@@ -0,0 +1,13 @@
+using QRCoder;
+
+namespace MusicBridge.Agent.Network;
+
+internal static class PairingQr
+{
+    public static byte[] Render(ConnectionInvitation invitation)
+    {
+        using var data = QRCodeGenerator.GenerateQrCode(invitation.ToQrPayload(), QRCodeGenerator.ECCLevel.M);
+        using var renderer = new PngByteQRCode(data);
+        return renderer.GetGraphic(6);
+    }
+}