id: PRICING-001
title: GLD Expense Ratio Decay Correction
status: backlog
priority: P0
size: S
depends_on: []
acceptance_criteria:
  - GLD weight_per_share uses the current expense-adjusted backing (~0.0919 oz/share in 2026), not the naive 0.1 oz/share
  - Expense ratio decay formula implemented: ounces_per_share = 0.10 * e^(-0.004 * years_since_2004)
  - All GLD conversions throughout the app use the corrected rate
  - Domain tests prove conversion math matches research document examples
  - Existing GLD-based tests updated to use corrected backing
notes:
  - See docs/GLD_BASIS_RESEARCH.md for the research basis
  - Current hardcoded 0.1 oz/share in app/domain/instruments.py is incorrect
  - This is a data/modeling fix, not a UI change
  - Should be implemented before any GC=F comparison features
implementation_hints:
  - Update app/domain/instruments.py to compute GLD backing dynamically
  - Add a data source for GLD NAV/ounce (can start with hardcoded 0.0919 approximation)
  - Verify app/domain/portfolio_math.py and app/domain/backtesting_math.py use corrected conversions