Positions now carry real valuation and risk: computed once, served everywhere

Positions gain valuation and risk fields, computed on the write side and served without any read-time price authority.

• usd_value, health_factor, borrow_limit, and liquidation_price materialized per position.
• A parity gate holds the health factor within ±0.001 of the Blend SDK over frozen ground-truth wallets, and it is price-sensitive.
• The API reads stored values only; it never prices a position itself.