Add Stage 2 package payload reader

[anth-volk]

Fixes #1073

Summary

• Add CalibrationPackagePayload, reader, and writer helpers for typed access to calibration_package.pkl while preserving legacy dictionary pickle compatibility.
• Route Stage 2 package summaries, geography summaries, persisted contract validation, and remote metadata sidecar writing through the typed payload boundary.
• Expose package payload read/write/checksum/sidecar failures as canonical ValidationReport records through CalibrationPackagePayloadError.
• Document the payload reader/writer nodes in the pipeline map and regenerated pipeline docs, with focused unit coverage for round trips, required keys, structured failures, legacy missing geography, summary parity, and remote sidecar generation.

Validation

• uv run --no-sync --with pyyaml python scripts/extract_pipeline_docs.py
• ruff check policyengine_us_data/calibration_package/__init__.py policyengine_us_data/calibration_package/payload.py tests/unit/calibration_package/test_payload.py tests/unit/test_calibration_package_stage_contract.py tests/unit/test_pipeline_docs_extractor.py
• ruff format --check policyengine_us_data/calibration_package/__init__.py policyengine_us_data/calibration_package/payload.py tests/unit/calibration_package/test_payload.py tests/unit/test_calibration_package_stage_contract.py tests/unit/test_pipeline_docs_extractor.py
• uv run --no-sync --with sqlmodel python -m pytest tests/unit/calibration_package/test_payload.py tests/unit/test_calibration_package_stage_contract.py tests/unit/test_pipeline_doc_guards.py tests/unit/test_pipeline_docs_extractor.py tests/unit/test_remote_calibration_runner.py (58 passed)
• uv run --no-sync --with pyyaml python scripts/run_quality_guards.py
• make lint
• git diff --check origin/main...HEAD
• git diff --check

Notes

• Compatibility with legacy dictionary pickles is intentional in PR 2c. The Stage 2 plan removes or isolates the legacy subprocess path later in PR 2j, after parity is proven.