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/index.ts

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+export { renderWithProviders, type RenderOptions } from "./render-with-providers.js";
+export { createMockTrpcClient } from "./mock-trpc.js";

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

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+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] });
+}

packages/core-testing/src/react/render-with-providers.test.tsx

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+import { describe, it, expect } from "vitest";
+import { screen } from "@testing-library/react";
+import { renderWithProviders } from "@/react/render-with-providers";
+
+describe("renderWithProviders", () => {
+  it("renders the child", () => {
+    renderWithProviders(<div data-testid="x">hi</div>);
+    expect(screen.getByTestId("x")).toBeInTheDocument();
+  });
+
+  it("returns the queryClient instance", () => {
+    const { queryClient } = renderWithProviders(<div />);
+    expect(queryClient).toBeDefined();
+  });
+});

packages/core-testing/src/react/render-with-providers.tsx

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+import type { PropsWithChildren, ReactElement } from "react";
+import { render } from "@testing-library/react";
+import type { RenderResult } from "@testing-library/react";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+
+export interface RenderOptions {
+  queryClient?: QueryClient;
+}
+
+export function renderWithProviders(
+  ui: ReactElement,
+  options: RenderOptions = {},
+): RenderResult & { queryClient: QueryClient } {
+  const queryClient =
+    options.queryClient ??
+    new QueryClient({
+      defaultOptions: {
+        queries: { retry: false },
+        mutations: { retry: false },
+      },
+    });
+
+  const Wrapper = ({ children }: PropsWithChildren) => (
+    <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
+  );
+
+  const result = render(ui, { wrapper: Wrapper });
+  return Object.assign(result, { queryClient });
+}