Add Arch-backed PE target parity adapters

[MaxGhenis]

Summary

• adds an Arch-backed target provider plus coverage/gap tooling for PE native target parity
• expands PE native broad target profiles and source adapters for SOI, TANF, LIHEAP, CMS, USDA, SSA, BEA, Census, Federal Reserve Z.1, KFF, and related publisher facts
• adds Census block population rollup support and helper scripts for target database hydration/refresh workflows
• depends on merged PolicyEngine/microplex core helper PR Make mp-3m-fast sparse, sharded, and profiled #15 instead of dirty sibling checkouts, and removes the unused microunit dependency
• adds tests around Arch fact mapping, target profile coverage, ACA PTC helpers, Census block rollups, and microdata role metadata

Paired Arch work

Merged Arch source-package PRs now backing this target set:

• Add Federal Reserve Z.1 net worth source package arch-data#26: Federal Reserve Z.1 household net worth
• Add CMS NHE Medicaid expenditure source package arch-data#27: CMS NHE Medicaid expenditure
• Add CMS Medicare Part B premium source package arch-data#28: CMS Medicare Part B premiums
• Add HHS ACF LIHEAP household count source package arch-data#29: HHS ACF LIHEAP household count
• Add national CMS Medicaid enrollment aggregate arch-data#30: national CMS Medicaid enrollment aggregate
• Add national KFF marketplace enrollment aggregate arch-data#31: national KFF marketplace effectuated enrollment aggregate

Current coverage

• current refreshed PE native broad coverage: 161 / 189 target cells covered
• national coverage: 94 / 116
• state coverage: 67 / 73
• latest coverage artifact: /Users/maxghenis/CosilicoAI/microplex-us/artifacts/arch_kff_national_marketplace_coverage_20260528/pe_native_broad_2024_coverage_y2023_plus_recent_arch_prs_with_stc_z1.json
• remaining uncovered cells are mostly concept-review items rather than safe direct-source imports: SALT/count splits, AGI-by-tax-before-credit slices, child care/support, rent/housing subsidy, pregnancy, SSN card type, and several medical premium/expense concepts.

Validation

• env -u UV_FROZEN uv lock --no-sources --dry-run -> no lockfile changes detected
• uv build -> built sdist and wheel
• uv run ruff check pyproject.toml src/microplex_us/targets/arch.py src/microplex_us/targets/census_blocks.py src/microplex_us/targets/aca_ptc.py src/microplex_us/targets/adapters.py src/microplex_us/geography.py src/microplex_us/microdata_roles.py tests/targets/test_arch.py tests/targets/test_arch_facts.py tests/targets/test_census_blocks.py tests/policyengine/test_target_profiles.py tests/targets/test_aca_ptc.py tests/test_microdata_roles.py
• uv run --extra dev --extra policyengine python -m pytest tests/targets/test_arch.py tests/targets/test_arch_facts.py tests/targets/test_census_blocks.py tests/policyengine/test_target_profiles.py tests/targets/test_aca_ptc.py tests/test_microdata_roles.py -q -> 107 passed
• uv run microplex-us-arch-target-coverage ... --profile pe_native_broad with the merged/recent Arch source suites -> 161 / 189 coverage