refactor(hierarchy): one Tree.Transitional, drop the duplicated cross-fade check

internal/hierarchy/hierarchy.go

@@ -272,6 +272,25 @@ func elementFromNode(node *treeNodeJSON) *Element {
 	return element
 }
 
+// Transitional reports more than one resource id ending in "Screen": the marker
+// of a Compose NavHost mid cross-fade, where the source and destination route
+// composables are both alive in a collapsed, mid-animation layout.
+func (t *Tree) Transitional() bool {
+	if t == nil {
+		return false
+	}
+	screens := 0
+	for _, element := range t.Elements {
+		if strings.HasSuffix(element.ResourceID, "Screen") {
+			screens++
+			if screens > 1 {
+				return true
+			}
+		}
+	}
+	return false
+}
+
 // Find returns the first element matching the selector, or nil.
 func (t *Tree) Find(selector string) *Element {
 	node := t.FindNode(selector)

internal/hierarchy/hierarchy_test.go

@@ -1006,3 +1006,32 @@ func TestParseBoundsRejectsBadInput(t *testing.T) {
 		}
 	}
 }
+
+// Bug class: a NavHost cross-fade carries two route-level *Screen ids at once;
+// both the runner's re-fetch guard and the LLM's candidate enumeration depend on
+// spotting it, and neither must flag a settled single-screen tree.
+func TestTreeTransitional(t *testing.T) {
+	multi, err := Parse(`{"attributes":{"resource-id":"root"},"children":[
+	  {"attributes":{"resource-id":"AddAccountScreen"},"children":[]},
+	  {"attributes":{"resource-id":"HomeScreen"},"children":[]}
+	]}`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !multi.Transitional() {
+		t.Error("expected multi-screen tree to be flagged as transitional")
+	}
+
+	single, err := Parse(`{"attributes":{"resource-id":"HomeScreen"},"children":[]}`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if single.Transitional() {
+		t.Error("single-screen tree must not be flagged as transitional")
+	}
+
+	var nilTree *Tree
+	if nilTree.Transitional() {
+		t.Error("nil tree must not be flagged as transitional")
+	}
+}