Add standard signal snapshots

[Pigbibi]

Summary

• add signal_snapshot.v1 payload builder
• attach signal snapshots to LongBridge notifications, runtime logs, and execution reports
• return the rebalance result from the orchestration layer for reporting

Tests

• .venv/bin/python -m pytest tests/test_rebalance_service.py tests/test_request_handling.py -q
• python -m py_compile application/signal_snapshot.py application/rebalance_service.py application/runtime_dependencies.py application/runtime_composer.py notifications/renderers.py main.py