docs: add GLD pricing and underlying selector roadmap items

docs/roadmap/backlog/PRICING-002-gld-gcf-basis-display.yaml

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+id: PRICING-002
+title: GLD/GC=F Basis Display
+status: backlog
+priority: P1
+size: M
+depends_on:
+  - PRICING-001
+acceptance_criteria:
+  - Overview page shows both GLD implied spot and GC=F adjusted for comparison
+  - Basis indicator visible with threshold-based coloring (green < 25bps, yellow 25-50bps, red > 50bps)
+  - Warning displayed when basis exceeds normal bounds
+  - After-hours pricing gap noted when GLD is stale vs GC=F
+  - Display "last regular session close" vs "current futures indication" where appropriate
+notes:
+  - GLD only trades US market hours (9:30 AM - 4 PM ET)
+  - GC=F trades 23 hours/day, 6 days/week
+  - Contango is typically ~$10/oz, causing GLD to appear at "discount" vs GC=F/10
+  - During stress (backwardation), GLD can appear at "premium"
+implementation_hints:
+  - Add basis calculation to price display logic
+  - Use GLD price / GLD_OUNCES_PER_SHARE for implied spot
+  - Use GC=F - contango_estimate for fair value comparison
+  - Consider adding a small info tooltip explaining basis