Create an integration test framework for validating Host–Workers contract

Create a test framework to write integration tests that validate the contract between the Host and Workers. The framework will provide essential helper utilities, including logic for starting the Host, handling placeholder and specialization flows, initializing all emulators, and so on. This framework will be used by both Host and Worker teams to write integration tests.

The issue names no files, tests, or entry points. First map the existing Host–Worker integration-test setup and emulator initialization, then define the framework’s scope around starting the Host and covering placeholder and specialization flows; done means both Host and Worker teams can use the helpers to validate their contract.

Written by the indexing model from the issue text.