Add 4.x output support for legacy intra_wealth_decile macro result

[anth-volk]

Context

policyengine-api-v2 needs a 4.x-supported way to produce the legacy single-year macro intra_wealth_decile section. Pre-1 Simulation.calculate_economy_comparison() produced this as part of the macro output. In policyengine 4.x, there is an IntraDecileImpact helper for income deciles, but I did not find a first-class helper for the legacy wealth-decile version.

This blocks a safe migration to 4.x while preserving the simulation API's public SingleYearMacroOutput contract.

Proposed work

Add a 4.x output helper for intra-wealth-decile impacts, using the same categories and semantics as the pre-1 macro output where possible.

Acceptance criteria

• Provide a .py output helper for intra_wealth_decile or a documented replacement.
• Preserve the legacy category semantics and output shape expected by API/report callers.
• Handle unsupported countries or missing wealth-decile variables explicitly.
• Add tests for normal output and unsupported/missing-data cases.