Accept Arch EITC child-count amount targets

[MaxGhenis]

Summary

• allow Arch sum targets to match a target-cell domain after removing the target variable self-domain
• add coverage regression for EITC amount by qualifying-child count
• raises Arch-backed broad target coverage with the companion Arch EITC source from 150/189 to 152/189

Validation

• uv run ruff check src/microplex_us/targets/arch.py tests/targets/test_arch_facts.py
• uv run --extra policyengine --extra dev python -m pytest tests/targets/test_arch_facts.py::test_arch_consumer_fact_jsonl_provider_maps_eitc_by_agi_and_children tests/targets/test_arch_facts.py::test_arch_consumer_fact_coverage_accepts_eitc_child_count_totals -q
• uv run --extra policyengine microplex-us-arch-target-coverage --arch-targets-db /Users/maxghenis/CosilicoAI/microplex-us/artifacts/arch_soi_eitc_child_coverage_20260528/arch_bundle/consumer_facts.jsonl --period 2024 --profile pe_native_broad --indent 2