Use family relationship for PE family units

[MaxGhenis]

Summary

• use CPS family_relationship as a family-membership signal when assigning PolicyEngine family units
• keep household-level SPM fallback and tax-unit construction unchanged
• add a regression test covering an other-relative who should stay in the primary family and a non-family household member who should remain separate

Real-data structural probe

Existing calibrated small ASEC+ACS100k artifact, no recalibration:

• families: 159,331 -> 109,234
• families per household: 1.59331 -> 1.09234
• family singleton share: 0.6567 -> 0.3795
• family cross-household count: 0
• tax units: unchanged at 130,980
• SPM units: unchanged at 100,000
• marital units: unchanged at 220,843
• filing-status counts: unchanged

Probe artifact: /Users/maxghenis/CosilicoAI/microplex-us/artifacts/small_asec_acs100k_family_relationship_units_20260529/structural_probe.json

This directly addresses the family overfragmentation part of #81. The resulting family count is slightly below eCPS, so this should still be followed by the sound eCPS comparison on a rematerialized H5 before calling it replacement-ready.

Tests

• 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 "family_relationship_for_family_units or household_level_spm_fallback or derives_relationships_from_family_relationship or derives_relationships_from_one_based_family_relationship or prefers_richer_family_relationship"
• uv run --python 3.13 --extra dev --extra policyengine pytest tests/pipelines/test_us.py

Refs #81