feat: grade liquidity share per PSA grade

[hjacque]

Goal

For each Product, determine how liquid each PSA Grade is — both relative to the card's own grade distribution and in absolute terms — so the Opportunity Score can distinguish between a real buying opportunity and a deal on a grade nobody actually trades.

Concept

Grade Liquidity Share: the fraction of a Product's total eBay sold volume at a specific PSA Grade over the Sold Comp Window.

Two dimensions both matter:

• Relative share — how does this grade's volume compare to the card's own distribution? (e.g. PSA 10 = 85% of volume vs PSA 7 = 2%)
• Absolute floor — a card with 3 total sales spread across grades is not liquid at any grade, regardless of distribution

Examples

Card	PSA 10 share	PSA 7 share	Signal
High-rarity card	25%	20%	Volume distributed → any grade is tradeable
Mid-rarity card	85%	2%	Volume concentrated → PSA 7 is illiquid, avoid
Any card with ≤ N total sales	—	—	Absolute floor not met → no grade is liquid

Design notes

• Exact thresholds (absolute floor count, share threshold for "liquid" vs "thin" vs "illiquid") require data calibration from real eBay sold history — do not hardcode
• Output: a liquidity label or score per grade (liquid / thin / illiquid) derived from the two dimensions above
• Feeds into the Opportunity Score in feat: fair value range per card #91 — a deal on an illiquid grade is discounted
• Separate signal from the Fair Value Range itself — liquidity affects confidence in the opportunity, not the price band

Blocked by

• feat: eBay sold listings as a Price Source #84 — requires eBay sold history to compute per-grade volume distribution