Refactor Starfold to adhere to DiffNode's invariants

The time specific children of `DiffNode` do not guarantee any ordering,
but the Starfold transformation requires these nodes to be ordered:
- When `respectNodeOrder` is active.
- For the label of the merged node, which is composed sequentially from
  the merged nodes in the order of the given children.

For that reason Starfold cannot use `DiffNode.getChildren` and has to
use `DiffNode.getAllChildren` instead. Because this list contains both
/before/ and /after/ children the actual computation has to filter the
children to a specific time.

It would also be possible to run the two cases (/before/ and /after/) in
parallel, but this a consumer with the semantic of a coroutine and a
finalisation step to merge the remaining nodes. But this introduces
complexity which doesn't seem adequate.

Author: ibbem

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

@@ -644,6 +644,18 @@ public Node getPresenceCondition(Time time) {
         );
     }
 
+    public boolean isBeforeChild(DiffNode child) {
+        return beforeChildren.contains(child);
+    }
+
+    public boolean isAfterChild(DiffNode child) {
+        return afterChildren.contains(child);
+    }
+
+    public boolean isChild(DiffNode child, Time time) {
+        return time.match(isBeforeChild(child), isAfterChild(child));
+    }
+
     public boolean isLeaf() {
         return childOrder.isEmpty();
     }

src/main/java/org/variantsync/diffdetective/diff/difftree/transform/Starfold.java

@@ -47,11 +47,13 @@ public void foldStar(final DiffNode starRoot) {
     public void foldStarAtTime(final DiffNode starRoot, Time time) {
 //        System.out.println("Fold " + starRoot + " at time " + time);
         final DiffType targetDiffType = DiffType.thatExistsOnlyAt(time);
-
-        final List<DiffNode> children = starRoot.getChildren(time);
         final List<DiffNode> starArms = new ArrayList<>();
 
-        for (DiffNode child : children) {
+        for (DiffNode child : starRoot.getAllChildren()) {
+            if (!starRoot.isChild(child, time)) {
+                continue;
+            }
+
             if (isStarArm(child) && child.diffType == targetDiffType) {
 //                System.out.println("  Found arm " + child);
                 starArms.add(child);