Google Cloud Dataflow adapter for the Culvert framework. Provides DataflowPipeline, a Pipeline implementation that holds the stage topology (per the Pipeline contract — name + stages + validate) plus utility methods for bridging to Apache Beam: buildBeam() translates the stage graph into a beam.sdk.Pipeline and runOnDataflow() submits it via the DataflowPipelineRunner. Sprint-2 deliverable (issue #25).