dsh-checkdigit: DeepSeek Harness Check-Digit Math Toolbox

`dsh-checkdigit` is a zero-dependency check-digit toolkit for DeepSeek Harness, designed to address the problem of large language models generating incorrect check digits. The plugin provides generation, validation, identification, and ISBN conversion capabilities, and supports 13 common schemes such as Luhn, Verhoeff, and IBAN. Users can install it via the official command and activate it in `dsh` to generate test data that complies with mathematical rules.