We do a little thieving

- Reimplement stealInstance.

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

@@ -24,6 +24,19 @@ public interface Instancer<I extends Instance> {
 	 */
 	I createInstance();
 
+	/**
+	 * Steal an instance from another instancer.
+	 * <br>
+	 * This has the effect of swapping the instance's model in-place.
+	 * <br><br>
+	 * If the given instance is already owned by this instancer, this method does nothing.
+	 * <br>
+	 * If the given instance was created by a different backend, the behavior of this method is undefined.
+	 *
+	 * @param instance The instance to steal.
+	 */
+	void stealInstance(I instance);
+
 	/**
 	 * Populate arr with new instances of this model.
 	 *

src/main/java/com/jozufozu/flywheel/backend/engine/AbstractInstancer.java

@@ -29,13 +29,51 @@ public abstract class AbstractInstancer<I extends Instance> implements Instancer
 			var handle = new InstanceHandleImpl(this, i);
 			I instance = type.create(handle);
 
-			instances.add(instance);
-			handles.add(handle);
-			changed.set(i);
+			addLocked(instance, handle);
 			return instance;
 		}
 	}
 
+	@Override
+	public void stealInstance(I instance) {
+		var instanceHandle = instance.handle();
+
+		if (!(instanceHandle instanceof InstanceHandleImpl handle)) {
+			// UB: do nothing
+			return;
+		}
+
+		if (handle.instancer == this) {
+			return;
+		}
+
+		// FIXME: in theory there could be a race condition here if the instance
+		//  is somehow being stolen by 2 different instancers between threads.
+		//  That seems kinda impossible so I'm fine leaving it as is for now.
+
+		// Remove the instance from its old instancer.
+		// This won't have any unwanted effect when the old instancer
+		// is filtering deleted instances later, so is safe.
+		handle.setDeleted();
+
+		// Only lock now that we'll be mutating our state.
+		synchronized (lock) {
+			// Add the instance to this instancer.
+			handle.instancer = this;
+			handle.index = instances.size();
+			addLocked(instance, handle);
+		}
+	}
+
+	/**
+	 * Calls must be synchronized on {@link #lock}.
+	 */
+	private void addLocked(I instance, InstanceHandleImpl handle) {
+		instances.add(instance);
+		handles.add(handle);
+		changed.set(handle.index);
+	}
+
 	public int getInstanceCount() {
 		return instances.size();
 	}

src/main/java/com/jozufozu/flywheel/backend/engine/InstanceHandleImpl.java

@@ -3,7 +3,7 @@ package com.jozufozu.flywheel.backend.engine;
 import com.jozufozu.flywheel.api.instance.InstanceHandle;
 
 public class InstanceHandleImpl implements InstanceHandle {
-	public final AbstractInstancer<?> instancer;
+	public AbstractInstancer<?> instancer;
 	public int index;
 
 	public InstanceHandleImpl(AbstractInstancer<?> instancer, int index) {