Compose SALT targets from Arch components

[MaxGhenis]

Summary

• derives Microplex Arch salt_amount targets from exact PolicyEngine SALT source components: state/local income-or-sales taxes plus real estate taxes
• leaves SOI total state/local taxes skipped because it includes personal property taxes outside the PolicyEngine salt variable definition
• preserves component source lineage and marks the composite as a Microplex component sum

Coverage

• source-backed coverage artifact: /Users/maxghenis/CosilicoAI/microplex-us/artifacts/arch_salt_component_composite_20260528/pe_native_broad_source_backed_2024_coverage_with_salt_composite.json
• source-backed coverage: 165 / 174, up from 164 / 174
• newly covered cell: national salt with domain_variable=salt,tax_unit_itemizes
• broad coverage artifact: /Users/maxghenis/CosilicoAI/microplex-us/artifacts/arch_salt_component_composite_20260528/pe_native_broad_2024_coverage_with_salt_composite.json
• broad coverage: 165 / 189

Tests

• /Users/maxghenis/.local/bin/uv run ruff check src/microplex_us/targets/arch.py tests/targets/test_arch_facts.py tests/targets/test_arch.py
• /Users/maxghenis/.local/bin/uv run --extra dev --extra policyengine python -m pytest tests/targets -q