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.

Media/TimelineMath.cs

@@ -0,0 +1,16 @@
+namespace MusicBridge.Agent.Media;
+
+internal static class TimelineMath
+{
+    public static double Position(double position, double start, double end, bool playing,
+        DateTimeOffset updatedAt, DateTimeOffset now, double rate)
+    {
+        if (playing && updatedAt > DateTimeOffset.UnixEpoch && updatedAt <= now && double.IsFinite(rate))
+            position += (now - updatedAt).TotalSeconds * rate;
+        position = Math.Max(start, position);
+        return end > start ? Math.Min(end, position) : position;
+    }
+
+    public static (double Min, double Max) SeekRange(double start, double end, double min, double max)
+        => (Math.Max(start, min), end > start ? Math.Min(end, max) : max);
+}