feat(features): contract suites for all repository interfaces

Each repository interface now has a contract suite under
src/__contracts__/. Both Mock and Payload implementations run the
same suite, eliminating mock-vs-real drift. Payload impls back the
contract with an in-memory stub via vi.mock('payload') + a small
buildPayloadStub helper.

Spec: §5.2, §6.4

packages/core-testing/package.json

@@ -9,6 +9,7 @@
     "./contract": "./src/contract/index.ts",
     "./react": "./src/react/index.ts",
     "./payload": "./src/payload/index.ts",
+    "./payload/stub-config": "./src/payload/stub-config.ts",
     "./setup/jsdom": "./src/setup/jsdom.ts",
     "./setup/node": "./src/setup/node.ts"
   },