dsh-stats-board: Session Statistics Dashboard for DeepSeek Harness Web

The DeepSeek Harness plugin `dsh-stats-board` adds a “Statistics” view for analyzing session data. By registering components, the plugin displays KPI overviews (such as round count, duration, token usage, and cache hit rate), a token composition donut chart, and round trend charts, while also providing a data details table. It relies on specific session nodes and projections to obtain data and renders directly under a no-build approach, aiming to offer developers a convenient data pivot tool.