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Problem

The LendingPool contract tracks deposits using raw token amounts per depositor. When yield is distributed via distribute_yield(), the total pool balance increases but individual depositor balances don't change until they withdraw.

This creates several issues:

1. Late depositors dilute early depositors — a new deposit after yield distribution gets proportional withdrawal rights without having earned the yield
2. Yield is socialized incorrectly — the test test_subsequent_depositor_does_not_dilute_existing_holders exists but the contract may not correctly handle all edge cases
3. No share price tracking — no way for the frontend to show accurate portfolio value including accrued yield

Expected Behavior

Implement share-based accounting (like ERC-4626 vault pattern):

• Deposits mint shares based on current share price
• Withdrawals burn shares and return proportional pool value
• Share price increases as yield accumulates

Files to Change

• contracts/lending_pool/src/lib.rs — implement share-based deposit/withdraw
• contracts/lending_pool/src/test.rs — update tests for share accounting

Acceptance Criteria

• deposit() mints shares based on amount / share_price
• withdraw() burns shares and returns shares * share_price
• get_share_price() query function returns current price
• get_shares(provider) returns share balance
• Yield distribution increases share price, not individual balances
• All existing tests updated to reflect share-based model