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/scenario.go

@@ -60,6 +60,18 @@ type Result struct {
 	ReachedPct     float64  `json:"reachedPct"`
 }
 
+// GraphEdges builds the world's social network from the config's graph
+// fields and returns its undirected edge list. The same config always
+// yields the same edges (seeded generator), so the API can expose
+// topology separately without every Result carrying it.
+func GraphEdges(config Config) ([][2]int, error) {
+	graph, err := HolmeKim(config.NumStudents, config.EdgesPerNode, config.TriangleProb, newRand(config.GraphSeed))
+	if err != nil {
+		return nil, err
+	}
+	return graph.Edges(), nil
+}
+
 // RunScenario builds the world the config describes (network plus edge
 // thresholds), picks the educated students per strategy, and runs the
 // cascade. Scenarios with the same config share the same world, so