Add Stage 2 target catalog selection

[anth-volk]

Fixes #1083

Summary

• Add a reusable Stage 2 target catalog/selection boundary with TargetCatalogReader, TargetCatalog, TargetSelectionPolicy, and TargetSelectionResult.
• Route target config filtering through the selection service and keep UnifiedMatrixBuilder as the matrix consumer via selected target metadata.
• Emit calibration_targets.jsonl and calibration_target_facets.json, include selection checksum metadata, and reference both artifacts from the Stage 2 contract and manifest output bundle.
• Document the target catalog/selection artifacts in the pipeline map and cover target rows, facets, disabled reporting, additive expressions, stable order/checksum, and fake fit joins.

Validation

• uv run --no-sync python -m py_compile policyengine_us_data/calibration_package/targets.py policyengine_us_data/calibration_package/specs.py policyengine_us_data/calibration_package/__init__.py policyengine_us_data/calibration/unified_calibration.py policyengine_us_data/calibration/unified_matrix_builder.py policyengine_us_data/stage_contracts/calibration_package.py
• uv run --no-sync ruff check policyengine_us_data/calibration/unified_calibration.py policyengine_us_data/calibration/unified_matrix_builder.py policyengine_us_data/calibration_package/__init__.py policyengine_us_data/calibration_package/specs.py policyengine_us_data/calibration_package/targets.py policyengine_us_data/stage_contracts/calibration_package.py tests/unit/calibration_package/test_specs.py tests/unit/calibration_package/test_targets.py tests/unit/test_calibration_package_stage_contract.py tests/unit/test_pipeline_docs_extractor.py
• uv run --no-sync ruff format --check policyengine_us_data/calibration/unified_calibration.py policyengine_us_data/calibration/unified_matrix_builder.py policyengine_us_data/calibration_package/__init__.py policyengine_us_data/calibration_package/specs.py policyengine_us_data/calibration_package/targets.py policyengine_us_data/stage_contracts/calibration_package.py tests/unit/calibration_package/test_specs.py tests/unit/calibration_package/test_targets.py tests/unit/test_calibration_package_stage_contract.py tests/unit/test_pipeline_docs_extractor.py
• uv run --no-sync python -m pytest tests/unit/calibration_package/test_targets.py tests/unit/calibration_package/test_specs.py tests/unit/test_calibration_package_stage_contract.py tests/unit/test_pipeline_docs_extractor.py (47 passed)
• uv run --no-sync python -m pytest tests/unit/calibration/test_target_config.py tests/unit/calibration/test_unified_matrix_builder.py::TestQueryTargets (39 passed)
• uv run --no-sync python -m pytest tests/unit/test_remote_calibration_runner.py tests/unit/test_pipeline_doc_guards.py tests/unit/calibration_package/test_payload.py tests/unit/build_outputs/test_geography_loader.py (24 passed, 1 skipped from tests/unit/test_pipeline.py when run separately due local Modal import skip)
• uv run --no-sync python -m pytest tests/unit/test_pipeline_source_contracts.py tests/unit/test_pipeline_docs_extractor.py tests/unit/test_pipeline_doc_guards.py (18 passed)
• uv run --no-sync --with pyyaml python scripts/extract_pipeline_docs.py --json /private/tmp/us-data-pr-2d-pipeline-docs/pipeline_map.json --api-json /private/tmp/us-data-pr-2d-pipeline-docs/pipeline_api.json --markdown /private/tmp/us-data-pr-2d-pipeline-docs/pipeline-map.md
• uv run --no-sync --with pyyaml python scripts/run_quality_guards.py
• git diff --check
• make lint

Notes

• The selector also emitted the full tests/unit/calibration/test_unified_calibration.py and several broader matrix/ETL/query tests. I ran the directly affected target-config and query-target tests plus the new target artifact/contract/pipeline docs coverage; broader full selector pytest was not run to avoid the known long collection/runtime behavior in this local environment.