fix: preserve the projection when splitting a DiffNode

Author: ibbem

src/main/java/org/variantsync/diffdetective/variation/diff/DiffNode.java

@@ -373,9 +373,12 @@ public DiffNode<L> split(Time time) {
         other.addChildren(this.removeChildren(time), time);
         getParent(time).replaceChild(this, other, time);
 
-        // FIXME? If we allowed access to the backing node,
-        // we could preserve the identity of this projection by moving it to `other`.
-        this.projections[time.ordinal()] = null;
+        // Preserve the projection by changing its `backingNode` to `other`.
+        if (this.projections[time.ordinal()] != null) {
+            other.projections[time.ordinal()] = this.projections[time.ordinal()];
+            this.projections[time.ordinal()] = null;
+            other.projections[time.ordinal()].backingNode = other;
+        }
 
         return other;
     }

src/main/java/org/variantsync/diffdetective/variation/diff/Projection.java

@@ -21,7 +21,16 @@
  * @see DiffNode#projection
  */
 public class Projection<L extends Label> extends VariationNode<Projection<L>, L> {
-    private DiffNode<L> backingNode;
+    /**
+     * The {@link DiffNode} which is projected at {@link time}.
+     * Invariant: {@code backingNode.projection(time) == this}
+     * <p>
+     * Note that this field is package private because {@link DiffNode} needs to change it,
+     * for example in {@link DiffNode#split},
+     * to preserve the identity of this instance and prevent it from becoming invalid.
+     * Only {@link DiffNode} is intended to have access to this field.
+     */
+    DiffNode<L> backingNode;
     private Time time;
 
     /**