Stabilize eCPS comparison diagnostics

[MaxGhenis]

Summary

• use the legacy eCPS contract entity map before length-based inference when loading PolicyEngine datasets
• let target diagnostics fall back per target to scaled values when PE-native scaling is zero
• add regressions for equal household/SPM lengths and zero-scaling diagnostics

Tests

• uv run --extra dev --extra policyengine pytest tests/policyengine/test_comparison.py -k "legacy_contract_entities or round_trips_written_dataset"
• uv run --extra dev --extra policyengine pytest tests/pipelines/test_ecps_replacement_comparison.py -k "zero_scaling or target_diagnostics_sidecar or satisfies_gate_contract"
• uv run --extra dev ruff check src/microplex_us/policyengine/us.py src/microplex_us/pipelines/ecps_replacement_comparison.py tests/policyengine/test_comparison.py tests/pipelines/test_ecps_replacement_comparison.py
• git diff --check