dsh-monitor-ppi: Monitor PPI Calculator

`dsh-monitor-ppi` is a Node.js-based utility plugin designed for DeepSeek Harness (DSH), used for local, precise calculation of monitor PPI. It has no network dependencies and can calculate and return detailed results when parameters are provided. Users can install it via a command and invoke it directly in DSH sessions. Ensure the required peer dependencies are satisfied before use.