Function luhnCheckDigit

luhnCheckDigit(body: string): number
Compute the Luhn check digit for a body of digits (without the check digit).

Returns the check digit (0-9) that, when appended, makes the full number Luhn-valid. Useful for generating synthetic test data or for the trailing digit on emitted document numbers.
Parameters
- body: string
  Digit-only string WITHOUT the check digit position.
Returns number
Integer in [0, 9] — the digit to append to make body Luhn-valid.
Throws
if body contains non-digit characters.
Example
```
import { luhnCheckDigit, luhnValid } from "nationid/algorithms";

const body = "7992739871";
const cd = luhnCheckDigit(body); // 3
luhnValid(body + String(cd));    // true
```
- Defined in algorithms/luhn.ts:64