Remove manual support sign controls from donor imputation

[MaxGhenis]

Summary

• replace sign-specific support families with a signless SUPPORT_SENSITIVE family
• remove DonorMatchStrategy.ZERO_INFLATED_POSITIVE and let rank matching auto-detect positive zero-inflated donor support
• remove RegimeAwareDonorImputer nonnegative/sign/rare-class knobs and stop clamping support-sensitive predictions in Microplex-US
• pin Microplex-US to Make zero-inflated regime detection presence-based microimpute#192 (27d23090) so ZeroInflatedImputer uses purely observed-support regime detection

Notes

• This PR depends on Make zero-inflated regime detection presence-based microimpute#192. Once that is released, the git SHA pin can be replaced with a released version.
• The legacy zi_qrf backend still has its own zero_inflated_vars/nonnegative_vars internals; the production regime_aware path no longer accepts or passes manual sign/clamp controls.

Tests

• uv run --python 3.13 --extra dev ruff check src/microplex_us/variables.py src/microplex_us/pipelines/us.py src/microplex_us/pipelines/donor_imputers.py tests/test_variables.py tests/pipelines/test_us.py tests/pipelines/test_zi_qrf_backend.py tests/pipelines/test_regime_aware_donor_imputer.py tests/data_sources/test_scf_net_worth_components.py
• uv run --python 3.13 --extra dev ruff format --check src/microplex_us/variables.py src/microplex_us/pipelines/us.py src/microplex_us/pipelines/donor_imputers.py tests/test_variables.py tests/pipelines/test_us.py tests/pipelines/test_zi_qrf_backend.py tests/pipelines/test_regime_aware_donor_imputer.py tests/data_sources/test_scf_net_worth_components.py
• uv run --python 3.13 --extra dev --extra policyengine python -m pytest tests/test_variables.py tests/pipelines/test_regime_aware_donor_imputer.py tests/pipelines/test_zi_qrf_backend.py tests/data_sources/test_scf_net_worth_components.py -q
• uv run --python 3.13 --extra dev --extra policyengine python -m pytest tests/pipelines/test_us.py -k "donor_imputer or support_sensitive or zero_inflated or signed_zero or clamp" -q
• git diff --check