dispatchEngineAction

Drive the engine. Each action either advances engineState or aborts the in-flight stream. Idempotent — calling ToolLoopAgentAction.Cancel when nothing is streaming is a no-op.

Renamed from onAction to disambiguate from host-specific orchestration action handler which takes a host-specific host action type. The port-level engine action dispatch lives here; subclasses layer their own onAction with different action types on top.