Update modelled_policies handling for programs.yaml structural changes

Context

policyengine-us#7716 updates the structure of policyengine_us/programs.yaml, changing the verified_years string field to separate verified_start_year/verified_end_year integers.

This matters for app-v2 because modelled_policies in the API metadata response is sourced directly from programs.yaml via the modelled_policies class attribute in policyengine_us/system.py:

modelled_policies = COUNTRY_DIR / "programs.yaml"

Current state in app-v2

• metadataUtils.ts:181 reads data.modelled_policies and normalises it into state with a safe fallback (?? { core: {}, filtered: {} })
• metadataReducer.ts stores it as state.modelledPolicies
• The type is defined in types/metadata.ts
• No UI component in app-v2 currently renders this data (unlike the old app, which had PoliciesModelledPopup.jsx)

What needs to happen

1. Update the modelled_policies TypeScript type in types/metadata.ts to reflect the new verified_start_year/verified_end_year integer fields once PR #7716 merges
2. Decide whether to implement a UI equivalent to the old app's PoliciesModelledPopup using this richer structured data, or keep it as unused state

References

• policyengine-us PR: https://github.com/PolicyEngine/policyengine-us/pull/7716
• Old app popup: PolicyEngine/policyengine-app → src/modals/PoliciesModelledPopup.jsx
• app-v2 consumer: app/src/libs/metadataUtils.ts:181, app/src/reducers/metadataReducer.ts:99

Start with types/metadata.ts, then trace app/src/libs/metadataUtils.ts:181 and app/src/reducers/metadataReducer.ts:99 to understand the current modelled_policies flow. Review policyengine-us PR #7716 and the old PoliciesModelledPopup.jsx reference; done means the TypeScript type matches the new fields and the issue's UI-versus-unused-state decision is resolved.