Add entity structure diagnostics to eCPS comparison

[MaxGhenis]

Summary

• add entity-structure diagnostics to the sound eCPS replacement comparison payload for source, matched, and refit datasets
• report per-entity unit counts, units per household, singleton shares, duplicate IDs, missing references, empty units, and cross-household units
• cover the new comparison contract fields in tests so family/SPM/tax-unit overfragmentation is visible in benchmark artifacts

Evidence

• Existing PR Use household-level fallback SPM units #80 artifact now shows Microplex matched families per household at 1.595 vs eCPS 1.119, with no duplicate/missing/cross-household family IDs
• This is diagnostic-only; it does not change H5 construction or scoring

Tests

• uv run --python 3.13 --extra dev ruff check src/microplex_us/pipelines/ecps_replacement_comparison.py tests/pipelines/test_ecps_replacement_comparison.py
• uv run --python 3.13 --extra dev --extra policyengine pytest tests/pipelines/test_ecps_replacement_comparison.py
• uv run --python 3.13 --extra dev --extra policyengine pytest tests/pipelines/test_mp300k_artifact_gates.py -k ecps

Refs #81