Coalesce sparse PE input aliases by row

[MaxGhenis]

Summary

• Coalesce sparse PE income/input aliases row-by-row when the canonical export column exists but is zero and a source alias carries the value.
• Applies to wage/employment income, self-employment income, taxable interest, dividend totals/components, and capital gains/LTCG inputs.
• Preserves component-first dividend behavior while allowing total-only rows to populate non-qualified dividends instead of being masked by all-zero components.

Context

The post-#170 H5 passed export-column parity, but source alias inspection showed important PE input families were being zeroed whenever a canonical column existed sparsely. The sidecar audit showed this particularly on employment/wages, taxable interest, and dividends. This PR fixes that structural alias masking without changing calibration, robust-loss scoring, or the APG optimizer branch.

Tests

• uv run --python 3.13 --extra dev python -m pytest tests/pipelines/test_us.py::TestUSMicroplexPipeline::test_augment_policyengine_person_inputs_coalesces_sparse_source_aliases_by_row tests/test_variables.py::test_normalize_dividend_columns_coalesces_sparse_total_aliases_by_row
• uv run --python 3.13 --extra dev python -m pytest tests/pipelines/test_us.py::TestUSMicroplexPipeline::test_augment_policyengine_person_inputs_materializes_agi_parity_inputs tests/pipelines/test_us.py::TestUSMicroplexPipeline::test_augment_policyengine_person_inputs_coalesces_sparse_source_aliases_by_row
• uv run --python 3.13 --extra dev python -m pytest tests/test_variables.py
• uv run --python 3.13 ruff check src/microplex_us/pipelines/us.py src/microplex_us/variables.py tests/pipelines/test_us.py tests/test_variables.py