fix: correct rate scaling, liquidity accounting, and code clarity issues

[Copilot]

Addressed unresolved review feedback regarding scaling inconsistencies, state management bugs, and code clarity issues in the lending protocol contracts.

Rate Scaling Fix

Problem: Interest calculation divided by YEAR_MS only, ignoring the 1e9 rate scaling factor. With base_rate = 10_000_000_000 (10% scaled), this calculated 10 billion percent interest instead of 10%.

Solution: Created INTEREST_DENOMINATOR = YEAR_MS × RATE_SCALE constant for proper scaling:

// Before: interest = borrowed × rate × elapsed / YEAR_MS
// After: interest = borrowed × rate × elapsed / (YEAR_MS × RATE_SCALE)
const INTEREST_DENOMINATOR: u128 = 31_557_600_000_000_000_000u128;
let interest = (total_borrowed as u128)
    .checked_mul(rate as u128)
    .and_then(|v| v.checked_mul(elapsed as u128))
    .and_then(|v| v.checked_div(INTEREST_DENOMINATOR))
    .unwrap_or(0) as Balance;

Liquidity Accounting Fix

Problem: disburse() decreased total_liquidity and receive_repayment() increased it, double-counting interest already added by accrue_interest().

Solution: total_liquidity now represents total pool value (available + borrowed). Only accrue_interest() modifies it:

• disburse(): increases total_borrowed, total_liquidity unchanged
• receive_repayment(): decreases total_borrowed, total_liquidity unchanged (interest already accrued)

Other Fixes

• DEFAULT_BOOST: Changed from 2_000_000_000 to 2 (raw star count, not scaled value)
• Removed underscore from purpose parameter with clarifying comment
• Fixed unused mut warning in disburse function
• Moved rate constants to module level

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