Split dense notification details

[Pigbibi]

Summary

• split cash summary, status/signal diagnostics, and benchmark rows into readable list lines
• keep holdings and execution logs in sectioned layout
• cover dense diagnostic layout in tests

Tests

• git diff --check
• .venv/bin/python -m pytest tests/test_rebalance_service.py tests/test_notifications.py -q
• .venv/bin/python -m pytest -q