Package dashboard and add Census SPM comparisons

[MaxGhenis]

Summary

• package the dashboard as a Python project with CI and Python 3.14 support
• add Census 2024 SPM report comparisons, threshold-ratio diagnostics, and source-replication controls
• add SPM element impact comparisons using arithmetic removal/addition without rerunning microsimulation
• add raw ASEC SPM total-income leaf diagnostics that reconstruct SPM_TOTVAL from public-use person income leaves and compare those leaves against ECPS concepts

Key diagnostic findings

• Raw CPS ASEC with Census-reported SPM resources remains close to Census, so the PE/Census gap is not primarily the threshold formula.
• Adding the missing child-support and workers-comp components closes only a small share of the gap.
• SPM_TOTVAL is exactly the SPM-unit sum of person PTOTVAL; public-use person income leaves reconstruct it essentially exactly.
• The largest SPM total-income leaf gaps are wage/salary income, pensions/annuities, farm self-employment, interest, retirement distributions, dividends, and Social Security.

Tests

• uv run pytest tests/test_spm_diagnostics.py tests/test_poverty_calc.py -q
• uv run ruff format --check poverty_dashboard/spm_diagnostics.py tests/test_spm_diagnostics.py
• uv run ruff check poverty_dashboard/spm_diagnostics.py tests/test_spm_diagnostics.py
• npm run typecheck in frontend/
• npm run build in frontend/