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/contract/define-contract-suite.test.ts

@@ -0,0 +1,27 @@
+import { describe, it, expect } from "vitest";
+import { defineContractSuite } from "@/contract/define-contract-suite";
+
+interface Adder {
+  add(a: number, b: number): number;
+}
+
+const adderContract = defineContractSuite<Adder>("Adder", ({ buildSubject }) => {
+  it("adds two positive numbers", async () => {
+    const subject = await buildSubject();
+    expect(subject.add(2, 3)).toBe(5);
+  });
+  it("handles zero", async () => {
+    const subject = await buildSubject();
+    expect(subject.add(0, 0)).toBe(0);
+  });
+});
+
+class RealAdder implements Adder {
+  add(a: number, b: number) {
+    return a + b;
+  }
+}
+
+describe("RealAdder satisfies Adder contract", () => {
+  adderContract.run(() => new RealAdder());
+});

packages/core-testing/src/contract/define-contract-suite.ts

@@ -0,0 +1,22 @@
+import { describe } from "vitest";
+
+export interface ContractContext<T> {
+  buildSubject: () => Promise<T> | T;
+}
+
+export interface ContractSuite<T> {
+  run(buildSubject: () => Promise<T> | T): void;
+}
+
+export function defineContractSuite<T>(
+  name: string,
+  suite: (ctx: ContractContext<T>) => void,
+): ContractSuite<T> {
+  return {
+    run(buildSubject) {
+      describe(`Contract: ${name}`, () => {
+        suite({ buildSubject });
+      });
+    },
+  };
+}

packages/core-testing/src/contract/index.ts

@@ -0,0 +1 @@
+export { defineContractSuite, type ContractContext, type ContractSuite } from "./define-contract-suite.js";