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/tsconfig.json

@@ -0,0 +1,12 @@
+{
+  "extends": "@repo/core-typescript/react-library.json",
+  "compilerOptions": {
+    "rootDir": ".",
+    "outDir": "dist",
+    "paths": {
+      "@/*": ["./src/*"]
+    }
+  },
+  "include": ["src/**/*", "vitest.config.ts"],
+  "exclude": ["node_modules", "dist"]
+}