Initial

Penumbra.Api/Api/IPenumbraApiBase.cs

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+namespace Penumbra.Api.Api;
+
+/// <summary> Base interface for the API that is always available, regardless of version. </summary>
+public interface IPenumbraApiBase
+{
+    /// <summary>
+    /// The API version is staggered in two parts.
+    /// The major/Breaking version only increments if there are changes breaking backwards compatibility.
+    /// The minor/Feature version increments any time there is something added
+    /// and resets when Breaking is incremented.
+    /// </summary>
+    public (int Breaking, int Feature) ApiVersion { get; }
+
+    /// <summary> Whether the API is still usable. </summary>
+    public bool Valid { get; }
+}