Write Stage 5 release promotion contract

[anth-volk]

Fixes #1037

Summary

• Add ReleasePromotionContractBuilder and helpers for Stage 5 release_promotion_contract.json.
• Write the contract during promote_run after the full promotion transaction succeeds and record it as a Stage 5 manifest output.
• Carry validation/diagnostics references into the production contract candidate, use typed promotion result paths for public refs, refresh generated pipeline docs, and bump policyengine-us to 1.703.2.

Validation

• ruff check modal_app/pipeline.py policyengine_us_data/release_promotion/__init__.py policyengine_us_data/release_promotion/contract.py policyengine_us_data/release_promotion/results/__init__.py policyengine_us_data/release_promotion/results/full.py tests/unit/release_promotion/test_contract.py tests/unit/release_promotion/test_results.py tests/unit/test_pipeline.py tests/unit/test_pipeline_source_contracts.py
• ruff format --check modal_app/pipeline.py policyengine_us_data/release_promotion/__init__.py policyengine_us_data/release_promotion/contract.py policyengine_us_data/release_promotion/results/__init__.py policyengine_us_data/release_promotion/results/full.py tests/unit/release_promotion/test_contract.py tests/unit/release_promotion/test_results.py tests/unit/test_pipeline.py tests/unit/test_pipeline_source_contracts.py
• uv run --no-sync pytest tests/unit/release_promotion/test_contract.py tests/unit/release_promotion/test_results.py tests/unit/test_pipeline_source_contracts.py tests/unit/test_pipeline_docs_extractor.py tests/unit/test_pipeline_doc_guards.py (40 passed)
• uv run --no-sync python .github/scripts/check_policyengine_us_dependency.py --mode fail
• uv lock --check
• uv run --no-sync --with pyyaml python scripts/extract_pipeline_docs.py
• git diff --check origin/main...HEAD

Note: tests/unit/test_pipeline.py remains Modal-gated locally; without modal installed, pytest collects 0 items / 1 skipped and exits with code 5. The subprocess-result parser is covered through the non-Modal Stage 5 result tests plus a source-contract test that ensures the Modal helper delegates to the pure parser.