id: CONV-001
title: Historical GLD Backing Lookup
status: backlog
priority: P1
size: S
depends_on:
  - PRICING-001
tags: [conversion, historical]
summary: Support lookups of GLD ounces-per-share for historical dates.
acceptance_criteria:
  - `gld_ounces_per_share(entry_date)` returns accurate backing for any date
  - Formula validates against known points (launch 2004, current 2026)
  - Conversion between GLD shares and gold oz uses entry_date backing
  - Error handling for dates outside GLD lifetime (pre-2004)
  - Cache-friendly for repeated lookups
notes:
  - GLD launched Nov 2004 with 0.10 oz/share
  - Current backing ~0.0916 oz/share (2026)
  - Formula: `0.10 × e^(-0.004 × years_since_2004)`
  - Future enhancement: fetch actual NAV/ounce from spdrgoldshares.com
implementation_hints:
  - Already implemented for current date in `app/domain/instruments.py`
  - Extend to accept `date` parameter
  - Clamp dates to GLD lifetime (Nov 2004 to present)
  - Add historical validation tests