docs: add instrument-aware quote unit story

docs/roadmap/backlog/CORE-002-instrument-aware-quote-units.yaml

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+id: CORE-002
+title: Instrument-Aware Quote Units and Conversion Multipliers
+status: backlog
+priority: P0
+effort: M
+depends_on:
+  - CORE-001A
+  - CORE-001B
+  - CORE-001C
+tags: [core, units, pricing, domain]
+summary: >
+  Model quoted instruments with explicit asset/share units and conversion multipliers
+  so prices like GLD are never misinterpreted as physical-gold ounce prices.
+acceptance_criteria:
+  - GLD quotes are modeled explicitly as prices per share, not as ambiguous gold-ounce prices.
+  - Instrument metadata encodes that 1 GLD share represents 0.1 troy ounces of gold exposure.
+  - Storage/domain value classes know the unit relationship and convert correctly when calculations combine instrument prices with ounce-based quantities.
+  - Cross-unit operations fail closed when no explicit instrument conversion path exists.
+  - Tests cover GLD share <-> troy-ounce conversion and reject unsupported mixed-unit operations.
+technical_notes:
+  - Prefer explicit instrument/unit types over ad hoc scale factors in page/service code.
+  - This should integrate with the Decimal/unit-safe domain model rather than bypass it with raw floats.
+  - Consider extending historical/unit types so both live overview paths and backtesting/event paths can share the same conversion rules.