Back me up here!

- Add @BackendImplemented annotation to API and use where needed.
- Add @ApiStatus.NonExtendable to interfaces that are missing it.

src/main/java/com/jozufozu/flywheel/api/BackendImplemented.java

@@ -0,0 +1,22 @@
+package com.jozufozu.flywheel.api;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * <p>Indicates that the annotated API class, interface or method must not be extended, implemented or overridden,
+ * <strong>except by backend implementations</strong>.</p>
+ *
+ * <p>API class, interface or method may not be marked {@code final} because it is extended by classes of registered backends
+ * but it is not supposed to be extended outside of backend implementations. Instances of classes and interfaces marked with this annotation
+ * may be cast to an internal implementing class within the active backend, leading to {@code ClassCastException}
+ * if a different implementation is provided by a client.</p>
+ */
+@Documented
+@Retention(RetentionPolicy.CLASS)
+@Target({ElementType.TYPE, ElementType.ANNOTATION_TYPE, ElementType.METHOD, ElementType.CONSTRUCTOR, ElementType.FIELD, ElementType.PACKAGE})
+public @interface BackendImplemented {
+}

src/main/java/com/jozufozu/flywheel/api/backend/Engine.java

@@ -2,6 +2,7 @@ package com.jozufozu.flywheel.api.backend;
 
 import java.util.List;
 
+import com.jozufozu.flywheel.api.BackendImplemented;
 import com.jozufozu.flywheel.api.event.RenderContext;
 import com.jozufozu.flywheel.api.event.RenderStage;
 import com.jozufozu.flywheel.api.instance.Instance;
@@ -13,6 +14,7 @@ import net.minecraft.client.Camera;
 import net.minecraft.core.BlockPos;
 import net.minecraft.core.Vec3i;
 
+@BackendImplemented
 public interface Engine extends InstancerProvider {
 	/**
 	 * Create a plan that will be executed every frame.

src/main/java/com/jozufozu/flywheel/api/event/RenderContext.java

@@ -1,5 +1,6 @@
 package com.jozufozu.flywheel.api.event;
 
+import org.jetbrains.annotations.ApiStatus;
 import org.joml.Matrix4f;
 
 import com.mojang.blaze3d.vertex.PoseStack;
@@ -9,6 +10,7 @@ import net.minecraft.client.multiplayer.ClientLevel;
 import net.minecraft.client.renderer.LevelRenderer;
 import net.minecraft.client.renderer.RenderBuffers;
 
+@ApiStatus.NonExtendable
 public interface RenderContext {
 	LevelRenderer renderer();
 

src/main/java/com/jozufozu/flywheel/api/instance/InstanceHandle.java

@@ -1,5 +1,8 @@
 package com.jozufozu.flywheel.api.instance;
 
+import com.jozufozu.flywheel.api.BackendImplemented;
+
+@BackendImplemented
 public interface InstanceHandle {
 	void setChanged();
 

src/main/java/com/jozufozu/flywheel/api/instance/Instancer.java

@@ -2,6 +2,8 @@ package com.jozufozu.flywheel.api.instance;
 
 import org.jetbrains.annotations.Nullable;
 
+import com.jozufozu.flywheel.api.BackendImplemented;
+
 /**
  * An instancer is how you interact with an instanced model.
  * <p>
@@ -20,6 +22,7 @@ import org.jetbrains.annotations.Nullable;
  *
  * @param <I> the data that represents a copy of the instanced model.
  */
+@BackendImplemented
 public interface Instancer<I extends Instance> {
 	/**
 	 * @return a handle to a new copy of this model.

src/main/java/com/jozufozu/flywheel/api/instance/InstancerProvider.java

@@ -1,8 +1,10 @@
 package com.jozufozu.flywheel.api.instance;
 
+import com.jozufozu.flywheel.api.BackendImplemented;
 import com.jozufozu.flywheel.api.event.RenderStage;
 import com.jozufozu.flywheel.api.model.Model;
 
+@BackendImplemented
 public interface InstancerProvider {
 	/**
 	 * Get an instancer for the given instance type, model, and render stage.

src/main/java/com/jozufozu/flywheel/api/visual/DistanceUpdateLimiter.java

@@ -1,8 +1,11 @@
 package com.jozufozu.flywheel.api.visual;
 
+import org.jetbrains.annotations.ApiStatus;
+
 /**
  * Interface for rate-limiting updates based on an object's distance from the camera.
  */
+@ApiStatus.NonExtendable
 public interface DistanceUpdateLimiter {
 	/**
 	 * Check to see if an object at the given position relative to the camera should be updated.

src/main/java/com/jozufozu/flywheel/api/visual/VisualFrameContext.java

@@ -1,7 +1,9 @@
 package com.jozufozu.flywheel.api.visual;
 
+import org.jetbrains.annotations.ApiStatus;
 import org.joml.FrustumIntersection;
 
+@ApiStatus.NonExtendable
 public interface VisualFrameContext {
 	double cameraX();
 

src/main/java/com/jozufozu/flywheel/api/visual/VisualTickContext.java

@@ -1,5 +1,8 @@
 package com.jozufozu.flywheel.api.visual;
 
+import org.jetbrains.annotations.ApiStatus;
+
+@ApiStatus.NonExtendable
 public interface VisualTickContext {
 	double cameraX();
 

src/main/java/com/jozufozu/flywheel/api/visualization/VisualizationContext.java

@@ -1,5 +1,7 @@
 package com.jozufozu.flywheel.api.visualization;
 
+import org.jetbrains.annotations.ApiStatus;
+
 import com.jozufozu.flywheel.api.instance.InstancerProvider;
 
 import net.minecraft.core.Vec3i;
@@ -7,6 +9,7 @@ import net.minecraft.core.Vec3i;
 /**
  * A context object passed on visual creation.
  */
+@ApiStatus.NonExtendable
 public interface VisualizationContext {
 	/**
 	 * @return The {@link InstancerProvider} that the visual can use to get instancers to render models.

src/main/java/com/jozufozu/flywheel/api/visualization/VisualizationLevel.java

@@ -6,7 +6,7 @@ import net.minecraft.world.level.LevelAccessor;
  * A marker interface custom levels can override to indicate
  * that block entities and entities inside the level should
  * render with Flywheel.
- *
+ * <br>
  * {@link net.minecraft.client.Minecraft#level Minecraft#level} is special cased and will support Flywheel by default.
  */
 public interface VisualizationLevel extends LevelAccessor {