Loan amortization math: fixed-rate monthly payment, per-period interest/principal split,
remaining balance, and lifetime interest. Pure Rust, no deps.
Same math behind the [AssetLoanCalculator](https://assetloancalculator.com/) amortization tool.
```rust
use amortization_schedule::{monthly_payment, total_interest};
let payment = monthly_payment(100_000.0, 0.07, 360); // ~665.30
let interest = total_interest(100_000.0, 0.07, 360); // lifetime interest
```
License: MIT