[codex] Add US equity runtime contracts and snapshot helpers

[Pigbibi]

Summary

• add explicit StrategyArtifactContract and StrategyRuntimePolicy contracts with validation and public exports
• add shared runtime config resolution and feature-snapshot evaluation helpers for broker platforms
• update US equity onboarding, cross-platform, and live-switch docs to define platform/strategy responsibilities and the current 8-profile switch matrix

Why

This gives the platform repos a typed contract for snapshot artifacts, config source policy, reconciliation output, and runtime execution windows, instead of relying on profile-name branches.

Validation

• PYTHONPATH=src .venv/bin/python -m unittest discover -s tests -v -> 115 passed
• /home/ubuntu/TelegramCodexCCBot/.venv/bin/ruff check tests/test_feature_snapshot_runtime.py tests/test_runtime_config.py -> passed
• Earlier focused check: PYTHONPATH=src .venv/bin/python -m pytest tests/test_feature_snapshot_runtime.py tests/test_runtime_config.py tests/test_strategy_contracts.py tests/test_strategies.py -> 51 passed

Merge Order

Merge this PR before the UsEquityStrategies and platform PRs that consume the new contracts.