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/AgentCertificate.cs

@@ -0,0 +1,25 @@
+using System.Net;
+using System.Security.Cryptography;
+using System.Security.Cryptography.X509Certificates;
+
+namespace MusicBridge.Agent.Network;
+
+internal static class AgentCertificate
+{
+    public static X509Certificate2 Create()
+    {
+        using var key = RSA.Create(2048);
+        var request = new CertificateRequest("CN=MusicBridge Agent", key, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
+        request.CertificateExtensions.Add(new X509BasicConstraintsExtension(false, false, 0, true));
+        request.CertificateExtensions.Add(new X509KeyUsageExtension(X509KeyUsageFlags.DigitalSignature | X509KeyUsageFlags.KeyEncipherment, true));
+        request.CertificateExtensions.Add(new X509EnhancedKeyUsageExtension(
+            new OidCollection { new("1.3.6.1.5.5.7.3.1") }, false));
+        var names = new SubjectAlternativeNameBuilder();
+        names.AddDnsName("localhost");
+        names.AddIpAddress(IPAddress.Loopback);
+        request.CertificateExtensions.Add(names.Build());
+        using var certificate = request.CreateSelfSigned(DateTimeOffset.UtcNow.AddMinutes(-5), DateTimeOffset.UtcNow.AddYears(5));
+        // Reload so Schannel can use the private key independently of the RSA object.
+        return X509CertificateLoader.LoadPkcs12(certificate.Export(X509ContentType.Pfx), null, X509KeyStorageFlags.Exportable);
+    }
+}