Fix nested inheritance history and budget accounting

internal/engine/inheritance_bench_test.go

@@ -58,6 +58,28 @@ func BenchmarkInheritanceInspectedLoad(b *testing.B) {
 	}
 }
 
+// Each scope has constant-sized output and evidence. This distinguishes budget
+// accounting overhead from legitimate growth in expanded parent payloads.
+func BenchmarkInheritanceIndependentScopes(b *testing.B) {
+	for _, size := range []int{256, 512, 1024} {
+		b.Run(strconv.Itoa(size), func(b *testing.B) {
+			input := &document.Node{Kind: document.Mapping}
+			for index := 0; index < size; index++ {
+				input.Mapping = append(input.Mapping, document.Entry{
+					Key: strconv.Itoa(index), Value: inheritanceBenchmarkInput("wide", 1),
+				})
+			}
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if _, diagnostics := expandInheritanceTree(input, defaultProjectionLimits()); len(diagnostics) != 0 {
+					b.Fatal(diagnostics)
+				}
+			}
+		})
+	}
+}
+
 func inheritanceBenchmarkInput(shape string, size int) *document.Node {
 	value := func(text string) *document.Node { return &document.Node{Kind: document.String, Text: text} }
 	mapping := func(entries ...document.Entry) *document.Node {