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/payload/mock-payload-module.ts

@@ -0,0 +1,10 @@
+import { vi } from "vitest";
+import type { Payload } from "payload";
+
+// Helper to mock the `payload` package with a custom getPayload impl.
+// Call inside a test file BEFORE importing the SUT.
+export function mockPayloadModule(impl: Partial<Payload>): void {
+  vi.mock("payload", () => ({
+    getPayload: vi.fn().mockResolvedValue(impl),
+  }));
+}