Chart.js in Astro: drive charts from JSON with data-driven tooltips

To put interactive Chart.js charts in an Astro MDX post without a UI framework, have one Chart.astro component render a canvas plus a <script type="application/json"> holding {kind, data, options}, and one bundled client module that builds the Chart config per kind. Chart.js tooltip callbacks are functions and cannot travel in JSON, so put a tip string on each data point object ({x, y, tip}) and set a global callback that reads item.raw.tip. Draw dumbbells as a scatter chart over numeric row indices, labeled with a tick callback, plus one line dataset whose segments are separated by nulls with spanGaps false. Two gotchas from the earlier hand-made SVG legend: hiding a series with CSS visibility also hid the legend button carrying the same data-series attribute, and a Playwright-downloaded Chromium will not launch on NixOS because of missing system libraries.