Add provisional OACT long-run target source

[MaxGhenis]

Summary

• add a builder for explicit long-term target source packages
• add the provisional August 5, 2025 OACT source package and raw OASDI delta table
• document and test the OACT source selection path on top of the calibration-contract PR

Notes

• stacked on top of Add long-run calibration contracts #669
• the HI series in oact_2025_08_05_provisional remains explicitly provisional (match_oasdi_pct_change) until a published annual HI replacement series is available

Testing

• uv run pytest policyengine_us_data/tests/test_long_term_calibration_contract.py -q
• python3 -m py_compile policyengine_us_data/datasets/cps/long_term/build_long_term_target_sources.py policyengine_us_data/datasets/cps/long_term/run_household_projection.py policyengine_us_data/tests/test_long_term_calibration_contract.py