feat: add IHasMultiviewControl interface for multiview enable and layout feedback

Exposes SetMultiviewEnabled plus MultiviewEnabled/MultiviewLayout feedbacks on an
Essentials Core interface so room plugins can drive a multiview/UVC compositor
(e.g. StreamSync) without a compile-time dependency on the hardware plugin. Layout
selection remains on IHasScreensWithLayouts.ApplyLayout.

src/PepperDash.Essentials.Core/DeviceTypeInterfaces/IHasMultiviewControl.cs

@@ -0,0 +1,28 @@
+namespace PepperDash.Essentials.Core.DeviceTypeInterfaces;
+
+/// <summary>
+/// Defines a device (e.g. a multiview/UVC compositor) whose multiview output can be enabled and
+/// whose active layout can be observed. Implementing this interface (instead of requiring consumers
+/// to reference the hardware-specific plugin type directly) lets other plugins (e.g. a room plugin)
+/// turn the multiview on and read its current layout without taking a compile-time dependency on the
+/// specific hardware plugin that implements it. The layout is <em>set</em> via
+/// <see cref="IHasScreensWithLayouts.ApplyLayout(uint, uint)"/>.
+/// </summary>
+public interface IHasMultiviewControl
+{
+    /// <summary>
+    /// Enables or disables the multiview output.
+    /// </summary>
+    /// <param name="enabled">True to enable multiview, false to disable.</param>
+    void SetMultiviewEnabled(bool enabled);
+
+    /// <summary>
+    /// Feedback for whether the multiview output is currently enabled.
+    /// </summary>
+    BoolFeedback MultiviewEnabled { get; }
+
+    /// <summary>
+    /// Feedback for the current multiview layout index.
+    /// </summary>
+    IntFeedback MultiviewLayout { get; }
+}