Restore US fiscal calibration targets

[MaxGhenis]

Summary

Closes #40.

This restores the US fiscal calibration target surface in Populace without reintroducing predecessor data-package dependencies. The package now ships a declarative fiscal target manifest with:

• total federal individual income tax from the 2024 CBO calibration parameter
• five JCT tax-expenditure rows encoded as simple neutralize-variable reform contracts
• 51 state/DC individual income-tax collection targets
• 400 SOI AGI/source distribution targets uprated to 2024

It also tightens target-profile gates so income_tax_positive cannot satisfy the total federal income-tax requirement, JCT rows must be precomputed income_tax(reform) - income_tax(baseline) rows rather than ordinary income_tax aggregates, and state income-tax coverage requires the actual 51-row state/DC surface.

Validation

• env -u UV_FROZEN uv run --python 3.13 ruff format --check .
• env -u UV_FROZEN uv run --python 3.13 ruff check .
• env -u UV_FROZEN uv run --python 3.13 --all-packages --all-groups python -m pytest packages/populace-build/tests/test_us_fiscal_targets.py packages/populace-build/tests/test_gates.py -q
• env -u UV_FROZEN uv run --python 3.13 --all-packages --all-groups python -m pytest -q
• git diff --check
• env -u UV_FROZEN uv build --package populace-build
• confirmed wheel and sdist include populace/build/us/fiscal_targets.json
• /cycle review: first pass found two gate loopholes; fixed and second pass found no actionable findings