docs: mark DATA-004 done, update roadmap

docs/roadmap/done/DATA-004-underlying-instrument-selector.yaml

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+id: DATA-004
+title: Underlying Instrument Selector
+status: backlog
+priority: P2
+size: L
+depends_on:
+  - PRICING-001
+acceptance_criteria:
+  - User can choose underlying instrument for options evaluation (GLD vs GC=F)
+  - Settings page exposes underlying selector with brief descriptions
+  - Workspace-scoped preference persisted
+  - Overview and Hedge pages reflect chosen underlying for price display and options strategy
+  - GC=F options data source placeholder exists (not yet wired to live data)
+  - UI clearly indicates which underlying is active and whether data is live or placeholder
+notes:
+  - GLD is the MVP supported underlying with live data via yfinance
+  - GC=F futures options will require a new data source (future roadmap item)
+  - For now, GC=F selection can show "coming soon" or use estimated/stubbed pricing
+  - Each underlying has different contract specs (100 shares vs futures contract multiplier)
+implementation_hints:
+  - Add underlying field to workspace/portfolio settings
+  - Extend app/domain/instruments.py with GC=F metadata (futures contract specs)
+  - Add underlying selector UI in settings
+  - DataService routes quote/options requests based on selected underlying
+  - For GC=F, stub with estimated pricing or clear "data source needed" state