feat(core-testing): scaffold shared testing utilities package

Adds @repo/core-testing (tag: tooling) with:
- factory/defineFactory: monotonic-sequence object factories with overrides
- contract/defineContractSuite: shared test suites runnable against multiple impls
- react/renderWithProviders + createMockTrpcClient: RTL helpers
- payload/stubPayloadConfig + mockPayloadModule: Payload mocking helpers
- setup/{jsdom,node}: vitest setup files

Spec: docs/superpowers/specs/2026-05-05-tdd-foundation-design.md §5

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/core-testing/src/react/mock-trpc.ts

@@ -0,0 +1,29 @@
+import { createTRPCClient, httpBatchLink } from "@trpc/client";
+import superjson from "superjson";
+import type { AnyTRPCRouter } from "@trpc/server";
+
+// Returns a tRPC client whose fetch is a stub honouring the provided mocks.
+// Mocks are keyed by procedure path ("blog.articleBySlug") returning the
+// raw response body.
+export function createMockTrpcClient<TRouter extends AnyTRPCRouter>(
+  mocks: Record<string, unknown> = {},
+) {
+  const fetchStub: typeof fetch = async (input) => {
+    const url = typeof input === "string" ? input : (input as Request).url;
+    const path = new URL(url, "http://mock").pathname.replace(/^\/api\/trpc\//, "");
+    const result = mocks[path];
+    if (result === undefined) {
+      return new Response(JSON.stringify([{ error: { code: -32603, message: `No mock for ${path}` } }]), { status: 200 });
+    }
+    return new Response(JSON.stringify([{ result: { data: superjson.serialize(result) } }]), { status: 200 });
+  };
+
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  const link = httpBatchLink<any>({
+    url: "http://mock/api/trpc",
+    transformer: superjson,
+    fetch: fetchStub,
+  });
+
+  return createTRPCClient<TRouter>({ links: [link] });
+}