Task: Scaffold story: Implement synthetic data generation against the new market data model

Open the story in Sprint 27 with a branch, a scaffold task and an implement task, so the synthetic data generation work has a home and the curve story has something concrete to wait on.

• The story and its two tasks exist, are wired into Sprint 27, and resolve from the index.
• The scaffold PR is open and recorded on this task.

Created the story and its tasks on feature/synthetic-data-against-market-data, wired them into Sprint 27, and opened PR #2703. The curve story now waits on this one.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

Every number this task claims about a tree, with the command that produced it and the commit it was measured at. A number without both is not a measurement, because a clean-up record describes a tree and trunk moves under a branch: a consumer count, a gate's component count and a suite's size all change with no code changing, and the reader of a bare number cannot tell which tree it came from. Where the raw output matters, put it in an evidence/ file beside the task and name that file in the Command column.

Manual QA scenarios (scaffolded via compass add test_scenario) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.