feat: Expand fraud detection to 30 rules + ST risk assessment

Major expansion of risk/financial_health.py from financial-report-minesweeper:

1. FraudDetector — 24 rules across 7 layers (唐朝 methodology):
   Layer 0: Audit opinion, disclosure timeliness
   Layer 1: Gross margin, other income, expense rate, impairments
   Layer 2: Operating CF, cash vs debt
   Layer 3: Receivables, inventory, construction in progress
   Layer 4: CFO/NI cross-validation, cash receipts, profit vs FCF
   Layer 5: Auditor changes, shareholder reduction, goodwill, penalties
   Layer 6: R&D capitalization

2. assess_st_risk() — ST delisting risk per 2024 exchange rules
   - Financial: consecutive losses, low revenue, negative equity

3. Original lightweight checks preserved as-is

1192 passed (all existing)

Author: sijie-Z