UK economy: constituency_impact and local_authority_impact changed from dict to list, breaking the maps #3955

Problem

UK economy results now return constituency_impact and local_authority_impact as lists, so the app's constituency and local authority maps show "No constituency data available" in production (v2 and v3).

GET /uk/economy/85546/over/1?region=uk&time_period=2026 (resolved_app_name: policyengine-simulation-py6-5-0, policyengine_bundle.model_version: 2.130.1):

• result.constituency_impact: a list of 632 objects {constituency_code, constituency_name, x, y, average_household_income_change, relative_household_income_change, population}
• result.local_authority_impact: a list of 363 objects with local_authority_code, local_authority_name and the same fields

The documented and previous shape (still emitted by the code in this repo) is a dict:

"constituency_impact": {
  "by_constituency": { "<name>": { "x": ..., "y": ..., "average_household_income_change": ..., "relative_household_income_change": ... } },
  "outcomes_by_region": { "england": { "Gain more than 5%": ..., ... }, ... }
}

The list shape also drops outcomes_by_region.

Request

1. Confirm which shape is the contract for /{country}/economy.
2. If the list is intended, document it and say whether outcomes_by_region is gone. If not, restore the dict shape.

App side

The app is being changed to accept both shapes (PolicyEngine/policyengine-app-v2, UK maps PR), so the maps work again either way.