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.go

@@ -48,3 +48,18 @@ func (g *Graph) Degree(u int) int { return len(g.adj[u]) }
 // Neighbors returns u's neighbours in insertion order. The slice is the
 // graph's own storage: callers must not modify it.
 func (g *Graph) Neighbors(u int) []int { return g.adj[u] }
+
+// Edges returns every undirected edge exactly once as a [from, to] pair
+// with from < to, in deterministic node order. The slice is freshly
+// allocated; callers may keep it.
+func (g *Graph) Edges() [][2]int {
+	edges := make([][2]int, 0, g.edges)
+	for node := range g.NumNodes() {
+		for _, neighbor := range g.adj[node] {
+			if node < neighbor {
+				edges = append(edges, [2]int{node, neighbor})
+			}
+		}
+	}
+	return edges
+}