Export defaulted PE contract inputs

[MaxGhenis]

Summary

• Adds tenure_type to the safe PE export surface now that Place derived contract inputs on PE entities #97 places it on households.
• Extends PE export defaults so variables like ssn_card_type are materialized even when the source column is absent, using the existing POLICYENGINE_US_EXPORT_DEFAULTS value.
• Adds regression coverage for absent default export maps and H5 array projection.

Evidence

• uv run --extra policyengine --python 3.13 ruff check src/microplex_us/policyengine/us.py tests/policyengine/test_us.py
• uv run --extra policyengine --python 3.13 pytest tests/policyengine/test_us.py -q
• Bounded contract probe: /Users/maxghenis/CosilicoAI/microplex-us/artifacts/export_contract_probe_20260529/contract_probe_export_defaults_subset.json
  • after Place derived contract inputs on PE entities #97: 92 / 185 contract columns
  • with this branch: 94 / 185 contract columns

Note: ruff format --check would reformat broader existing sections of these files; this PR intentionally keeps the diff scoped to behavior and tests.