WorkflowInterfaceEntry.turn docs point at isKnownTurn, which core does not export #48

The docblock on WorkflowInterfaceEntry.turn (lib/types/index.ts:1545) says "Narrow with isKnownTurn before indexing by it". isKnownTurn (and validateWorkflowInterface) are not exported from @flowdrop/flowdrop/core, so a consumer cannot follow that advice.

Fix: either export isKnownTurn from core next to WORKFLOW_INTERFACE_INPUT_TURNS / WORKFLOW_INTERFACE_OUTPUT_TURNS, or change the docblock to point at those exported constants.