api: POST /api/scenario returns result plus graph topology (M3 slice 1)

One panel's whole world in one call: effective config in, echoed
config + cascade result + undirected edge list out. Edges are
[from, to] pairs with from < to in deterministic node order;
Graph.Edges() walks the adjacency once, GraphEdges(config) rebuilds
the seeded world (~25us) so Result stays lean and /api/comparison
stays untouched. This closes the topology gap the design brief
flagged; the frontend's seeded d3-force layout consumes these pairs.
Go bits: [][2]int is a slice of fixed-size arrays; [2]int is a value
type, comparable, and JSON-marshals to [a, b], exactly the wire
shape the spec asks for.
tygo regen includes a fix: engine.Strategy now maps to the generated
Strategy type instead of decaying to 'any' in ScenarioRequest.
Verified live through the dev stack: 7/120 reached, 351 edges.

internal/engine/graph_test.go

@@ -74,3 +74,56 @@ func TestGraphNeighborsAndDegree(t *testing.T) {
 		t.Error("HasEdge(1, 2) = true, want false")
 	}
 }
+
+func TestGraphEdgesListsEveryEdgeOnce(t *testing.T) {
+	graph := NewGraph(4)
+	graph.AddEdge(2, 1) // insertion order must not matter; pairs come out from < to
+	graph.AddEdge(0, 1)
+	graph.AddEdge(3, 0)
+
+	want := [][2]int{{0, 1}, {0, 3}, {1, 2}}
+	if got := graph.Edges(); !slices.Equal(got, want) {
+		t.Errorf("Edges() = %v, want %v", got, want)
+	}
+}
+
+func TestGraphEdgesMatchesGeneratedGraph(t *testing.T) {
+	graph, err := HolmeKim(60, 3, 0.45, newRand(17))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	edges := graph.Edges()
+	if len(edges) != graph.NumEdges() {
+		t.Fatalf("len(Edges()) = %d, want NumEdges() = %d", len(edges), graph.NumEdges())
+	}
+	for _, edge := range edges {
+		from, to := edge[0], edge[1]
+		if from >= to {
+			t.Errorf("edge %v: want from < to", edge)
+		}
+		if to >= graph.NumNodes() {
+			t.Errorf("edge %v: endpoint out of range", edge)
+		}
+		if !graph.HasEdge(from, to) {
+			t.Errorf("edge %v not present in adjacency", edge)
+		}
+	}
+}
+
+func TestGraphEdgesDeterministicFromConfig(t *testing.T) {
+	first, err := GraphEdges(DefaultConfig())
+	if err != nil {
+		t.Fatal(err)
+	}
+	second, err := GraphEdges(DefaultConfig())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !slices.Equal(first, second) {
+		t.Error("GraphEdges() differs across identical configs")
+	}
+	if len(first) == 0 {
+		t.Error("GraphEdges() returned no edges")
+	}
+}