Advance Funnel Stage

Advance the funnel stage from the client side.

Studio calls this when key milestones happen in-browser that the server cannot observe directly (e.g. first_search executed in the test tab). Accepts both resource-creation stages and activation funnel stages.

source is optional provenance for the stage being advanced, recorded on the analytics event so a stage reached two different ways can be counted separately. It distinguishes a click on sample data from a real upload, and a search of a sample retriever from a search of the caller's own data, which the stage name alone cannot. Ignored for resource-creation stages.