Function mrzCheckDigit

Public algorithm primitives. Tree-shakable subpath: nationid/algorithms.

Each function is a pure mathematical primitive with cited spec.

mrzCheckDigit(input: string): number
Compute the MRZ check digit for a string per ICAO 9303 (cyclic weights [7, 3, 1], mod 10).

Used as a building block to validate the document number, DOB, expiry, and the final composite check digit of an MRZ line.
Parameters
- input: string
  A string drawn from the MRZ alphabet ([0-9A-Z<]*).
Returns number
The check digit in [0, 9]. Empty string returns 0.
Throws
ICAO_INVALID_CHAR if input contains any character outside the MRZ alphabet.
Example
```
import { mrzCheckDigit } from "nationid/algorithms";

mrzCheckDigit("L898902C<"); // 3  (ICAO 9303 worked example)
```
- Defined in algorithms/icao-9303.ts:69