Use canonical regime-gated microimpute API

[MaxGhenis]

Summary

• switch the regime-aware donor adapter from the removed microimpute.models.zero_inflated import to canonical microimpute.Imputer
• keep sign-regime gating explicit and preserve the QRF tree/train-sample budget wrapper
• make the regime-aware tests require the canonical microimpute API instead of skipping when zero_inflated is absent

Why

The 1c2e1e9 SSA-split candidate build failed at the first donor block because startup guards checked only version/backend fields while RegimeAwareDonorImputer.fit still imported microimpute.models.zero_inflated. The pinned canonical microimpute branch exports microimpute.Imputer from microimpute.models.regime_gated, not zero_inflated.

Local checks

• UV_PROJECT_ENVIRONMENT=.venv313 env -u PYTHONPATH -u UV_FROZEN uv run --python 3.13 --extra dev --extra policyengine python -m pytest tests/pipelines/test_regime_aware_donor_imputer.py -q
• UV_PROJECT_ENVIRONMENT=.venv313 env -u PYTHONPATH -u UV_FROZEN uv run --python 3.13 --extra dev --extra policyengine python -m pytest tests/pipelines/test_regime_aware_donor_imputer.py tests/pipelines/test_pe_us_data_rebuild.py tests/pipelines/test_pe_us_data_rebuild_checkpoint.py tests/data_sources/test_scf_net_worth_components.py -q
• UV_PROJECT_ENVIRONMENT=.venv313 env -u PYTHONPATH -u UV_FROZEN uv run --python 3.13 --extra dev --extra policyengine ruff check src/microplex_us/pipelines/donor_imputers.py tests/pipelines/test_regime_aware_donor_imputer.py