Explain small-account cash substitution

[Pigbibi]

Summary

• use shared small-account cash compatibility notes from QuantPlatformKit
• add zh/en Telegram buy-note text when an unbuyable risk target keeps cash instead of rebuying BOXX
• pin QuantPlatformKit to ceb84a3

Tests

• PYTHONPATH=/home/ubuntu/Projects/QuantPlatformKit-small-cash/src:. python -m pytest tests/test_rebalance_service.py -k 'strategy_target_keeps_cash_when_only_risk_target_is_unbuyable or target_gap_below_one_share_does_not_report_cash_shortage or safe_haven_target_below_cash_substitute_threshold_stays_cash' -q
• ruff check application/execution_service.py notifications/telegram.py tests/test_rebalance_service.py
• py_compile targeted files