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/vitest.config.ts

@@ -0,0 +1,16 @@
+import path from "node:path";
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: "jsdom",
+    include: ["src/**/*.test.{ts,tsx}"],
+    setupFiles: ["./src/setup/jsdom.ts"],
+    clearMocks: true,
+    restoreMocks: true,
+  },
+  resolve: {
+    alias: { "@": path.resolve(__dirname, "./src") },
+  },
+});