Add migration contract harness

[anth-volk]

Fixes #3636

Summary

Adds the PR 1 migration safety harness for the API v2 migration plan without changing production behavior by default.

Changes

• Adds typed migration route metadata and no-op source/traffic flags.
• Adds structured migration-context request logging to the Flask app.
• Adds typed app-v2 workflow contract registry and API v1 contract tests.
• Adds current simulation gateway contract tests.
• Adds opt-in baseline capture tooling for API route request metrics and simulation completion metrics.
• Adds a PR 1 baseline runbook and changelog entry.

Testing

• python -m pytest tests/contract -q -> 20 passed
• FLASK_DEBUG=1 python -m pytest tests/unit/test_migration_flags.py tests/unit/routes/test_migration_context_logging.py tests/unit/test_capture_migration_baseline.py tests/unit/libs/test_simulation_api_modal.py tests/contract -q -> 82 passed, 1 existing warning
• ruff format --check policyengine_api/api.py policyengine_api/migration_registry.py policyengine_api/migration_flags.py policyengine_api/migration_logging.py scripts/capture_migration_baseline.py tests/unit/test_migration_flags.py tests/unit/routes/test_migration_context_logging.py tests/unit/test_capture_migration_baseline.py tests/contract -> passed
• ruff check policyengine_api/migration_registry.py policyengine_api/migration_flags.py policyengine_api/migration_logging.py scripts/capture_migration_baseline.py tests/unit/test_migration_flags.py tests/unit/routes/test_migration_context_logging.py tests/unit/test_capture_migration_baseline.py tests/contract -> passed

Notes

This PR does not shift traffic, add FastAPI, add Cloud Run deployment, migrate schema/storage, or change the public API contract.

[codecov]

Codecov Report

❌ Patch coverage is 87.82609% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.66%. Comparing base (9b249e5) to head (61e0fac).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
policyengine_api/migration_flags.py	87.83%	5 Missing and 4 partials ⚠️
policyengine_api/migration_logging.py	88.88%	2 Missing and 1 partial ⚠️
policyengine_api/migration_registry.py	81.81%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3637      +/-   ##
==========================================
+ Coverage   78.01%   78.66%   +0.65%     
==========================================
  Files          67       70       +3     
  Lines        4071     4186     +115     
  Branches      759      776      +17     
==========================================
+ Hits         3176     3293     +117     
+ Misses        700      691       -9     
- Partials      195      202       +7     

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