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/LocalAddresses.cs

@@ -0,0 +1,17 @@
+using System.Net;
+using System.Net.NetworkInformation;
+
+namespace MusicBridge.Agent.Network;
+
+internal static class LocalAddresses
+{
+    public static IPAddress[] Read() => NetworkInterface.GetAllNetworkInterfaces()
+        .Where(nic => nic.OperationalStatus == OperationalStatus.Up && nic.SupportsMulticast
+            && nic.NetworkInterfaceType != NetworkInterfaceType.Loopback)
+        .SelectMany(nic => nic.GetIPProperties().UnicastAddresses)
+        .Select(a => a.Address)
+        .Where(IsEligible).Distinct().OrderBy(a => a.ToString(), StringComparer.Ordinal).ToArray();
+
+    public static bool IsEligible(IPAddress ip) => ip.AddressFamily == System.Net.Sockets.AddressFamily.InterNetwork
+        && ConnectionInvitation.IsLocalEndpoint(new Uri($"https://{ip}"));
+}