Migrate household handling to API v2 alpha structures

[anth-volk]

Fixes #621

Summary

• Migrates the app's internal household representation from API v1 format to API v2 Alpha format
• Introduces legacyConversion.ts as an anti-corruption layer at the API boundary (v1↔v2 conversion) so all code below the API layer is pure v2
• Rewrites types, adapter, utilities, hooks, components, tests, and fixtures for array-based entities with flat values and numeric IDs
• Cleans up all dead deprecated types, fixes broken type guards, removes vestigial _year parameter threading

What changed

Layer	Changes
Types	New Household interface with tax_benefit_model_name, year, people: HouseholdPerson[], array-based entity groups
Anti-corruption	legacyConversion.ts — 4 call sites in src/api/ and hooks; to be deleted when API v2 endpoints ship
Adapter	HouseholdAdapter updated for v2 format
Utilities	HouseholdBuilder, HouseholdQueries, HouseholdValidation, householdValues, VariableResolver, and others rewritten for arrays
Hooks	useCreateHousehold, useUserHousehold, useHouseholdVariation updated
Components	HouseholdBuilderForm, VariableInput, VariableRow, entity display components updated
Cleanup	Deleted 8 dead deprecated types, fixed isHousehold/isGeography type guards, removed _year prop threading, deleted dead v1 test fixtures and mock JSON

Requires API v2 alpha changes

This PR introduces a legacyConversion.ts anti-corruption layer because the API v1 endpoints are still live. New endpoints must be created in policyengine-api-v2-alpha to accept and return the v2 household format. Once those endpoints are available, legacyConversion.ts and its 4 call sites can be deleted entirely.

Test plan

• TypeScript: 0 type errors (npx tsc --noEmit)
• Tests: 2713 passed, 0 failures (npx vitest run)
• Manual: verify household builder form creates/edits households correctly
• Manual: verify report output pages render household data correctly

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Project	Deployment	Actions	Updated (UTC)
policyengine-app-v2	Ready	Preview, Comment	Feb 2, 2026 7:01pm
policyengine-calculator	Ready	Preview, Comment	Feb 2, 2026 7:01pm
policyengine-website	Ready	Preview, Comment	Feb 2, 2026 7:01pm