refactor: implement deterministic budget allocation in SoftTopkStrategy

[feedseawave]

Description

This PR refactors the SoftTopkStrategy to address the non-deterministic weight allocation issue during portfolio rebalancing.

Motivation and Context

• Fix Portfolio Overflow Logic: Addressed the issue where final_stock_weight was incorrectly handled when the number of held stocks exceeded topk (common when trade_impact_limit prevents immediate liquidation of old positions).
• Priority-based Allocation: Implemented a "COMPLIANCE_FIRST" or "IMPACT_FIRST". budget logic.
• Synchronization: Buy orders are now synchronized with the actual "freed budget" from sells, maintaining the target risk degree ($1 - \text{cash}$) more accurately.
• Code Cleanup: Improved code readability by aligning comments and standardizing variable naming conventions

How Has This Been Tested?

• Pass the test by running: pytest qlib/tests/test_all_pipeline.py under upper directory of qlib.
• If you are adding a new feature, test on your own test scripts.

Screenshots of Test Results (if appropriate):

1. Pipeline test:
2. Your own tests:

Types of changes

• Fix bugs
• Add new feature
• Update documentation

[feedseawave]

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