Add Stage 2 package identity specs

[anth-volk]

Fixes #1041

Summary

• add Stage 2 calibration-package specs for artifact paths and target-config identity
• include target-config path, checksum, and mode in Stage 2 reuse and contract parameters
• record calibration_package_contract.json as a durable Stage 2 manifest output and document the new Stage 2 waypoints

Validation

• uv run --no-sync python -m py_compile policyengine_us_data/calibration_package/specs.py policyengine_us_data/calibration/unified_calibration.py policyengine_us_data/stage_contracts/calibration_package_schema.py policyengine_us_data/stage_contracts/calibration_package.py modal_app/pipeline.py modal_app/remote_calibration_runner.py
• uv run --no-sync ruff format --check ...
• uv run --no-sync ruff check ...
• uv run --no-sync pytest tests/unit/calibration_package/test_specs.py tests/unit/test_calibration_package_stage_contract.py tests/unit/test_pipeline_docs_extractor.py tests/unit/test_pipeline_source_contracts.py
• uv run --no-sync pytest tests/unit/test_remote_calibration_runner.py tests/unit/test_pipeline_doc_guards.py
• uv run --no-sync --with pyyaml python scripts/run_quality_guards.py
• make lint

Note: the full selector-emitted pytest bundle was attempted locally, but this macOS x86_64 environment cannot complete the full dependency path cleanly (uv sync --dev is blocked by the torch==2.9.1 wheel/platform constraint) and the selector bundle then stalled during collection after manually adding missing local dependencies. The focused changed-file unit tests above passed.