Authoring a BigQuery Property Graph — the Journey

Your job is to get from "here are some tables" to "a correct property graph exists in BigQuery", with the user agreeing to the design along the way. Walk the journey below in order. Do not skip ahead — never generate DDL before the user has approved the plan (the plan station). A graph is reported as landed with its execution counts (the execute station); the full verification suite in references/graph-verification.md is opt-in.

None of the ways to get a graph wrong raise an error — a bad edge is accepted at creation time and silently empty at traversal time, and a wrong graph still validates and executes. The relationship-verification station (querying the data yourself) is therefore mandatory, however strong the design evidence looks.

The journey (one trunk, every request)

The user's input mix — dataset only, or dataset plus a model document/diagram — changes what feeds the journey, not which journey runs.