Fixes #1065

Summary

• add Stage2InputBundle, CalibrationPackageOutputBundle, and run-scoped Stage 2 build context specs
• resolve Stage 2 inputs from dataset_build_output.json when available, while keeping filename fallback marked as compatibility-only
• validate Stage 1 contract-declared input checksum and size before package work starts
• wire pipeline and remote package construction through the input/output bundles before package work starts
• add structured input validation material to the Stage 2 manifest identity and update pipeline docs metadata

Validation

• uv run --no-sync --with pyyaml python scripts/extract_pipeline_docs.py
• ruff check modal_app/pipeline.py modal_app/remote_calibration_runner.py policyengine_us_data/calibration_package/__init__.py policyengine_us_data/calibration_package/specs.py tests/unit/calibration_package/test_specs.py tests/unit/test_pipeline_docs_extractor.py tests/unit/test_pipeline_source_contracts.py
• ruff format --check modal_app/pipeline.py modal_app/remote_calibration_runner.py policyengine_us_data/calibration_package/__init__.py policyengine_us_data/calibration_package/specs.py tests/unit/calibration_package/test_specs.py tests/unit/test_pipeline_docs_extractor.py tests/unit/test_pipeline_source_contracts.py
• uv run --no-sync --with pyyaml python scripts/run_quality_guards.py
• uv run --no-sync pytest tests/unit/calibration_package/test_specs.py tests/unit/test_pipeline.py tests/unit/test_pipeline_doc_guards.py tests/unit/test_pipeline_docs_extractor.py tests/unit/test_pipeline_source_contracts.py tests/unit/test_remote_calibration_runner.py (42 passed, 1 skipped)
• make lint
• git diff --check origin/main...HEAD
• git diff --check