Document crypto live pool authority

[Pigbibi]

Summary:

• Document that CryptoSnapshotPipelines owns monthly crypto live-pool membership, ranking, and order.
• Clarify that live_pool.json and artifact_manifest.json are the stable downstream execution contract.
• Update operator guidance so downstream systems preserve the published symbol order.

Tests:

• timeout 120 env PYTHONPATH=/home/ubuntu/Projects/CryptoSnapshotPipelines /tmp/crypto-snapshot-test-venv/bin/python -m unittest tests.test_release_contract -v
• Result: OK after temporary binary-wheel install of pandas/numpy/PyYAML/requests in /tmp.
• Snapshot smoke was attempted and stopped at missing matplotlib to avoid expanding into heavier pipeline dependencies for doc-only changes.