docs: add GLD pricing and underlying selector roadmap items

docs/roadmap/backlog/PRICING-001-gld-expense-ratio-decay.yaml

@@ -0,0 +1,21 @@
+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