Add Stage 2 geography assignment boundary

[anth-volk]

Fixes #1086

Summary

• Add GeographyAssignmentSpec and GeographyAssignmentResult as the Stage 2 geography assignment boundary, including deterministic input identity for seed, household AGI, district AGI targets, and fixed state overrides.
• Emit geography_assignment_summary.json with clone-major block, county, state, and congressional district availability, lengths, unique counts, checksums, ordering, status, and spec material.
• Reference the geography summary artifact from the Stage 2 output bundle, runtime manifest outputs, and calibration package contract while preserving existing package pickle geography keys.
• Update pipeline docs and focused tests for summary JSON, contract validation, deterministic assignment, row-count/order checks, target/package coverage, and tiny Stage 4 geography loading.

Validation

• uv run --no-sync python -m py_compile policyengine_us_data/calibration_package/geography.py policyengine_us_data/calibration_package/payload.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/stage_contracts/calibration_package.py policyengine_us_data/stage_contracts/calibration_package_schema.py
• uv run --no-sync ruff check policyengine_us_data/calibration/unified_calibration.py policyengine_us_data/calibration_package/__init__.py policyengine_us_data/calibration_package/geography.py policyengine_us_data/calibration_package/payload.py policyengine_us_data/calibration_package/specs.py policyengine_us_data/stage_contracts/calibration_package.py policyengine_us_data/stage_contracts/calibration_package_schema.py tests/unit/calibration_package/test_geography.py tests/unit/calibration_package/test_specs.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_package/__init__.py policyengine_us_data/calibration_package/geography.py policyengine_us_data/calibration_package/payload.py policyengine_us_data/calibration_package/specs.py policyengine_us_data/stage_contracts/calibration_package.py policyengine_us_data/stage_contracts/calibration_package_schema.py tests/unit/calibration_package/test_geography.py tests/unit/calibration_package/test_specs.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_geography.py tests/unit/calibration_package/test_payload.py tests/unit/calibration_package/test_specs.py tests/unit/test_calibration_package_stage_contract.py tests/unit/test_pipeline_docs_extractor.py (56 passed)
• uv run --no-sync python -m pytest tests/unit/build_outputs/test_geography_loader.py (8 passed)
• uv run --no-sync python -m pytest tests/unit/calibration/test_target_config.py tests/unit/calibration_package/test_targets.py tests/unit/test_pipeline_doc_guards.py tests/unit/test_remote_calibration_runner.py tests/unit/test_pipeline.py (43 passed, 1 skipped)
• uv run --no-sync python -m pytest tests/unit/calibration/test_unified_calibration.py::test_calibration_package_contract_parameters_track_effective_matrix_mode tests/unit/calibration/test_unified_calibration.py::test_calibration_package_contract_parameters_ignore_unused_chunk_options tests/unit/calibration/test_unified_calibration.py::TestForbesStateOverrides tests/unit/calibration/test_unified_calibration.py::TestGeographyAssignmentCountyFips tests/unit/calibration/test_unified_calibration.py::TestRunCalibrationAgiTargets::test_uses_requested_db_for_district_agi_targets (7 passed)
• uv run --no-sync --with pyyaml python scripts/extract_pipeline_docs.py --json /private/tmp/us-data-pr-2e-pipeline-docs/pipeline_map.json --api-json /private/tmp/us-data-pr-2e-pipeline-docs/pipeline_api.json --markdown /private/tmp/us-data-pr-2e-pipeline-docs/pipeline-map.md (148 decorated objects)
• uv run --no-sync --with pyyaml python scripts/run_quality_guards.py
• git diff --check
• make lint

Notes

• The selector emitted the full tests/unit/calibration/test_unified_calibration.py. I ran the directly geography/package-related node IDs from that file; a collect-only probe for the full file took about 66 seconds and the selected seven tests took about 2:19, so the whole file was not run to avoid the known heavy local collection/runtime behavior.