Move the shell/fragment broadcast protocol into the core #41

Part of the core extraction (#35). Move the shell/fragment broadcast protocol into the core.

Scope

• The broadcast sink, host, log provider and wire messages move to the core; the Angular classes become thin @Injectable subclasses.
• A sink and a log provider can share one connection; enabled: false and dispose() stop sending.

Done when

• The wire fixture from the safety-net issue passes unchanged, for every pairing of an Angular or core shell with an Angular or core fragment.