Remove policyengine-us-data runtime imports

[MaxGhenis]

Summary

• move MP take-up/WIC inputs into MP-owned constants instead of importing policyengine_us_data.parameters
• make PUF pre-tax contribution imputation fall back to MP CPS ASEC training data instead of spawning/importing policyengine-us-data
• demote nonzero pe_l0 to an explicit experimental adapter requiring an injected fitter; zero-L0 stays on the local dense no-gate path
• add an AST regression guard that fails on direct policyengine_us_data imports in src/microplex_us

Notes

This removes production runtime imports of policyengine_us_data from MP package code. Benchmark/comparison/parity entrypoints can still accept explicit eCPS H5/repo paths because MP still needs to compare against eCPS while replacing it.

L0 is intentionally not part of the current production MP/eCPS path. The active eCPS replacement builds use entropy/no-L0; nonzero L0 now requires deliberate experimental injection rather than implicit policyengine-us-data loading.

Tests

• uv run ruff check src/microplex_us/pipelines/us.py tests/pipelines/test_us.py src/microplex_us/pipelines/pe_l0.py tests/pipelines/test_pe_l0.py src/microplex_us/data_sources/puf.py tests/test_puf_source_provider.py tests/test_no_policyengine_us_data_runtime_imports.py
• uv run ruff format --check src/microplex_us/pipelines/us.py tests/pipelines/test_us.py src/microplex_us/pipelines/pe_l0.py tests/pipelines/test_pe_l0.py src/microplex_us/data_sources/puf.py tests/test_puf_source_provider.py tests/test_no_policyengine_us_data_runtime_imports.py
• uv run pytest tests/pipelines/test_us.py tests/pipelines/test_pe_l0.py tests/test_puf_source_provider.py tests/test_no_policyengine_us_data_runtime_imports.py -q -k "(policyengine_entity_tables and (takeup or wic or stochastic or child_count)) or pe_l0 or pre_tax_contributions or no_policyengine_us_data"
• AST scan over src/microplex_us/**/*.py for direct policyengine_us_data imports