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/DiscoveryDescriptor.cs

@@ -0,0 +1,19 @@
+namespace MusicBridge.Agent.Network;
+
+// Data for a future DNS-SD publisher. Does not open sockets or advertise pairing secrets.
+internal sealed record DiscoveryDescriptor(int ProtocolVersion, string AgentId, string ServiceType, string InstanceName)
+{
+    public static DiscoveryDescriptor Create(string certificateSha256)
+    {
+        if (certificateSha256.Length != 64 || !certificateSha256.All(Uri.IsHexDigit))
+            throw new ArgumentException("Неверный идентификатор сертификата.");
+        var id = certificateSha256.ToUpperInvariant();
+        return new(1, id, "_musicbridge._tcp.local.", "MusicBridge-" + id[..12]);
+    }
+
+    public IReadOnlyDictionary<string, string> TxtRecords() => new Dictionary<string, string>
+    {
+        ["v"] = ProtocolVersion.ToString(System.Globalization.CultureInfo.InvariantCulture),
+        ["id"] = AgentId
+    };
+}