billing-kit 0.1.0

Billing primitives — Price with tax/discount, re-exporting decimal-money CurrencyAmount
Documentation 
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# billing-kit

Billing primitives on top of [`decimal-money`](https://github.com/WyattAu/money).

- Re-exports `Currency`, `CurrencyAmount`, `MoneyError`
- `Price` — net + tax_rate% (VAT), with `gross()`, `tax_amount()`, `discount_of_gross()`, `gross_minus_discount()`

## Quick start

```rust
use billing_kit::{Price, Currency};
use rust_decimal_macros::dec;

let p = Price::new(dec!(100), Currency::GBP, dec!(20)).unwrap();
assert_eq!(p.gross().amount, dec!(120));
```

## Assessment vs ecom-engine

`ecom-core::Money` now wraps `CurrencyAmount` (GBP) internally; `billing-kit` is **not required** for the GBP migration (which preserved `Money(Decimal)` storage + `sqlx(transparent)`). Scaffold this crate when extracting promo/insurance/VAT math from `ecom-http` into a shared billing crate.