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>

tsconfig.base.json

@@ -25,7 +25,14 @@
       "@repo/marketing-pages/api": ["packages/marketing-pages/src/integrations/api/router.ts"],
       "@repo/navigation": ["packages/navigation/src/index.ts"],
       "@repo/navigation/cms": ["packages/navigation/src/integrations/cms/index.ts"],
-      "@repo/navigation/api": ["packages/navigation/src/integrations/api/router.ts"]
+      "@repo/navigation/api": ["packages/navigation/src/integrations/api/router.ts"],
+      "@repo/core-testing": ["packages/core-testing/src/index.ts"],
+      "@repo/core-testing/factory": ["packages/core-testing/src/factory/index.ts"],
+      "@repo/core-testing/contract": ["packages/core-testing/src/contract/index.ts"],
+      "@repo/core-testing/react": ["packages/core-testing/src/react/index.ts"],
+      "@repo/core-testing/payload": ["packages/core-testing/src/payload/index.ts"],
+      "@repo/core-testing/setup/jsdom": ["packages/core-testing/src/setup/jsdom.ts"],
+      "@repo/core-testing/setup/node": ["packages/core-testing/src/setup/node.ts"]
     }
   }
 }