Write Stage 5 promoted runs index

[anth-volk]

Fixes #1066

Summary

• add typed PromotedRunsIndex helpers for calibration/runs/index.json, keyed by canonical run_id
• update the promoted runs index after the Stage 5 promotion transaction succeeds, including already-finalized marker-verified reruns
• reference promoted_runs_index and its update status from release_promotion_contract.json, with docs and unit/source-contract coverage

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/promoted_runs_index.py tests/unit/release_promotion/test_contract.py tests/unit/release_promotion/test_promoted_runs_index.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/promoted_runs_index.py tests/unit/release_promotion/test_contract.py tests/unit/release_promotion/test_promoted_runs_index.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_promoted_runs_index.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 (27 passed, 1 skipped; local Modal-gated tests/unit/test_pipeline.py skipped)
• make lint

Review

• Consolidated local review found no remaining high/critical in-scope issues.