Done — implemented in [app/api/topology/route.ts](../app/api/topology/route.ts), serves fixture data from [lib/clusterMock.ts](../lib/clusterMock.ts). Mock only: no real cluster, no 256 KB cap logic (fixture is small), `truncated` always `false`. See [REQUIREMENTS.md](REQUIREMENTS.md) for the real endpoint's contract.
Remaining: wire [app/cluster/page.tsx](../app/cluster/page.tsx) to fetch this instead of importing `clusterMock` directly.