Rename payroll_tax to employee_payroll_tax in economic impact analysis

[PavelMakarchuk]

Summary

• Renames payroll_tax to employee_payroll_tax in the economic impact analysis programs dict to match the correct variable name.

Test plan

• Verify economic impact analysis runs correctly with the renamed variable
• Check that payroll tax figures are included in analysis output

🤖 Generated with Claude Code

[MaxGhenis]

Closing per stale-PR policy: conflicting with main since April. If the rename is still wanted it is a small fresh PR against the current program-statistics mapping (see also #326).

[MaxGhenis]

Reopening pending a proper per-PR review; specific disposition to follow.

[MaxGhenis]

Closing after review — superseded by #327 (merged 2026-05-06), which landed this rename and more: main's US_PROGRAMS now maps employee_payroll_tax and medicare_cost, derives each program's entity from variable metadata at runtime (so the hardcoded entity keys this branch edits no longer exist), and the two docs files touched here were replaced in the v4 docs rewrite. The rename itself was semantically right — employee_payroll_tax is the correct employee-share TaxUnit variable — it just already happened. The durable mapping design remains tracked in #326.