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.

tygo.yaml

@@ -11,7 +11,8 @@ packages:
     include_files:
       - "api.go"
     frontmatter: |
-      import type { Config, Result } from "./engine";
+      import type { Config, Result, Strategy } from "./engine";
     type_mappings:
       engine.Config: "Config"
       engine.Result: "Result"
+      engine.Strategy: "Strategy"