Lockstep release of all 7 @scenar/* packages (first publish of @scenar/stubs).

Highlights:
- core: embed postMessage protocol (v1) + createEmbedHostController, a
  framework-free driver for an embedded player.
- react: embed-side runtime for the packed-scenario iframe.
- cli: scenar pack/deploy; emits a responsive embed snippet on deploy.
- stubs: generated protobuf-es stubs published as a first-class package so
  @scenar/cli installs cleanly (previously a dangling runtime dependency).

Internal:
- release: guard test asserts the publish set covers every public workspace
  package, preventing dangling-dependency releases.
- preview: MSW worker is written directly (no npx), making init deterministic.