Function luhnValid

luhnValid(digits: string): boolean
Luhn algorithm — ISO/IEC 7812-1.

Used by: credit cards, CA SIN, FR SIREN/SIRET, IT P.IVA, IL TZ/HP, ZA ID, and several IDs that document the algorithm as "Luhn variant".

Algorithm:
1. From the rightmost digit, double every second digit.
2. If doubling produces a 2-digit number, sum its digits (or subtract 9).
3. Sum all digits.
4. Number is valid iff sum is divisible by 10.
Public-domain mathematical algorithm.
Parameters
- digits: string
  Digit-only string including the trailing check digit. Non-digit input is rejected (returns false) — separators must be stripped by the caller.
Returns boolean
true when the digits satisfy the Luhn property (sum mod 10 = 0).
Example
```
import { luhnValid } from "nationid/algorithms";

luhnValid("79927398713"); // true  (canonical Luhn specimen)
luhnValid("79927398710"); // false
luhnValid("12-34");       // false (non-digit characters)
```
- Defined in algorithms/luhn.ts:28