Lift the four stable extension interfaces into Fallout.Core

src/Fallout.Build/Execution/Extensibility/BuildExtensionAttributeBase.cs

@@ -4,11 +4,6 @@
 
 namespace Fallout.Common.Execution;
 
-public interface IBuildExtension
-{
-    float Priority { get; }
-}
-
 [AttributeUsage(AttributeTargets.Class)]
 public abstract class BuildExtensionAttributeBase : Attribute, IBuildExtension
 {
@@ -52,8 +47,3 @@ public interface IOnTargetFailed : IBuildExtension
 {
     void OnTargetFailed(ExecutableTarget target);
 }
-
-public interface IOnBuildFinished : IBuildExtension
-{
-    void OnBuildFinished();
-}

src/Fallout.Core/Execution/IBuildExtension.cs

@@ -0,0 +1,18 @@
+namespace Fallout.Common.Execution;
+
+/// <summary>
+/// Root marker for a build extension — the stable seam the plugin SDK builds lifecycle
+/// listeners on. Higher <see cref="Priority"/> runs earlier.
+/// </summary>
+public interface IBuildExtension
+{
+    float Priority { get; }
+}
+
+/// <summary>
+/// Build extension invoked once the build has finished (whatever the outcome).
+/// </summary>
+public interface IOnBuildFinished : IBuildExtension
+{
+    void OnBuildFinished();
+}