Regression testing and better issue tracking for generated geometry

With some of the new attempts at crossing geometry, it feels like playing whack-a-mole -- problems in one area are fixed, but another area might get worse. I've used some form of "snapshot / diff testing" in other projects -- capture raw OSM input in ideally small areas, record GJ of the generated geometry in git, and then for new code changes, just see what changes and decide visually whether it's a net win or loss. Something like that would be useful here, but setting this up conveniently is tedious:

- From within Speedwalk, I want to just zoom in somewhere and press a button to add a test case. It should probably remember the sequence of bulk actions I've taken (usually "generate sidewalks for all roads, including small roads; generate imaginary crossings not just on major roads; connect crossings and include crossing=no")
- I want to run a test script and have ideally a single browser page that shows before/after renderings of anything that changed

该 issue 描述了为 Speedwalk 中生成的几何图形添加回归测试。首先探索代码库中现有的测试基础设施，可能位于 tests/ 目录中。查找当前如何生成和渲染几何图形，可能是在 Svelte 组件或 JavaScript 模块中。目标是在 UI 中实现一个按钮，用于捕获 OSM 输入和生成的 GeoJSON，并编写一个用于 diff 和可视化变更的脚本。检查项目中是否已经使用了任何 snapshot 或 diff 测试库。

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