Derive SSTB QBI qualification export

[MaxGhenis]

Summary

Closes the small G9 eCPS export gap for sstb_self_employment_income_would_be_qualified.

• derive the column from imputed business_is_sstb when present
• add a PolicyEngine-US default fallback so the required eCPS contract column is exported even when source rows lack the SSTB flag
• extend pipeline/export-map regressions

This is intentionally separate from PR #125 (G7 CPS income copies) and PR #126 (CPS/SPM observed inputs).

Validation

• uv run --extra dev python -m pytest tests/pipelines/test_us.py::TestUSMicroplexPipeline::test_build_policyengine_entity_tables_derives_tax_input_columns tests/policyengine/test_us.py::TestPolicyEngineUSProjection::test_build_policyengine_us_export_variable_maps_includes_contract_inputs -q
• uv run --extra dev ruff check src/microplex_us/pipelines/us.py src/microplex_us/policyengine/us.py tests/pipelines/test_us.py tests/policyengine/test_us.py

Release gate

Do not use this PR alone as evidence that Microplex can replace eCPS. The release path still requires a fresh CPS-spine rebuild and sound eCPS comparison/reform/repro checks after the relevant gap PRs land.

[MaxGhenis]

Closing as superseded by PR #127, which owns the G6/G8/G9 lane including SSTB-QBI and unmarried-partner recodes. Keeping PR #126 trimmed to the distinct CPS/SPM housing/Medicare takeup inputs.