bug(pool): withdraw yield calculation uses single cumulative LPInitialDeposit, inaccurate for multi-deposit LPs

[K1NGD4VID]

Summary

The withdraw function at contracts/pool/src/lib.rs:290-293 tracks LP principal as a single cumulative value (LPInitialDeposit) rather than per-deposit basis. For LPs who have made multiple deposits at different share prices, a partial withdrawal computes an incorrect principal portion and yield.

Current Behavior

let principal_portion = shares * init_dep / (lp_shares);
let yield_earned = usdc_to_return.saturating_sub(principal_portion);

Where init_dep is the sum of all deposits this LP ever made. For an LP depositing 100 at share price 1.0 and 100 at share price 1.5, init_dep = 200 but lp_shares = 166. A partial withdrawal of 50 shares computes principal_portion = 50 * 200 / 166 = 60, overstating principal and understating yield.

Expected Behavior

Track per-deposit lots (FIFO or HIFO) for accurate yield calculation, or clearly document the approximation and its limitations.

Acceptance Criteria

• Analysis of the impact on multi-deposit LPs
• Either fix the calculation or document the limitation
• Tests demonstrate the error margin for realistic multi-deposit scenarios

Tech Stack

contracts/pool/src/lib.rs function withdraw (lines 238-317)