Use household-level fallback SPM units

[MaxGhenis]

Summary

• assign one fallback SPM unit per household in the PolicyEngine entity-table builder
• keep the existing family fallback split unchanged for now
• add a regression test that an unrelated adult stays in a separate family while sharing the household SPM unit

Evidence

• uv run --python 3.13 --extra dev ruff check src/microplex_us/pipelines/us.py tests/pipelines/test_us.py
• uv run --python 3.13 --extra dev --extra policyengine pytest tests/pipelines/test_us.py -k 'spm_unit or tax_unit_role_flags or role_conflicts or missing_role_flag_heads or young_head_hint_dependents or positive_income_adult_heads'\n- uv run --python 3.13 --extra dev --extra policyengine pytest tests/pipelines/test_us.py::TestUSMicroplexPipeline::test_build_policyengine_entity_tables_uses_household_level_spm_fallback\n- uv run --python 3.13 --extra dev --extra policyengine pytest tests/pipelines/test_us.py\n\n## Structural probe\nUsing the existing calibrated small dataset at /Users/maxghenis/CosilicoAI/microplex-us/artifacts/small_asec_acs100k_ssi_disability_model_fixed_sipp_status_20260524/gd_l0zero_e2000_latest_us_data/calibrated_data.parquet, the new builder writes /Users/maxghenis/CosilicoAI/microplex-us/artifacts/small_asec_acs100k_household_spm_20260529/structural_probe.json with:\n\n- households: 100,000\n- persons: 245,714\n- tax units: 130,980, unchanged from the household-coherent tax-unit build\n- SPM units: 100,000, down from the prior 159,331 family-mirrored fallback\n- families: 159,331, intentionally unchanged in this PR\n\nThis leaves the richer family-unit question for a separate change while removing the worst SPM overfragmentation from the PE export fallback.