Impact Mode: Refine the visualisation

Besides just studying the impact on shortcuts within one neighbourhood boundary, the user can see how traffic changes across roads in the whole map. This works by finding the best route before and after changes for every origin/destination "OD" pairs, then counting routes per road.

Features:

- [x] Demand model mvp: random OD pairs
- [x] Demand model v2: Use same NPW data for Scotland. Need to detect country and decide how to clip that data. For those special areas, prebake maps with everything bundled in? Or prebake clipped OD datasets that match map names?
- [x] Just do the calculation naively
- [x] Display relative results
- [ ] Experiment with showing absolute vs relative counts
- [ ] Let user change the main road penalty
- [ ] Change default main road penalty to > 1.0 ?
- [x] Click one road to see paths through it that changed
- [ ] Consider CSV export of the data (being careful about communicating the absolute vs relative change)

Not important, just a stray idea: Consider some kind of differential "spider map"; see https://github.com/Urban-Analytics-Technology-Platform/od2net/issues/35

该 issue 描述了增强“Impact Mode”可视化的需求，以显示整个地图上的交通变化。首先检查现有的需求模型和路线计算代码，这些代码可能位于处理 OD pairs 和道路网络的模块中。查找当前的可视化逻辑，以了解相对结果是如何显示的。任务包括试验绝对计数与相对计数、调整 main road penalty，以及实现 click-to-see-paths 功能。检查是否已有与 impact mode 相关的测试或示例，以了解当前行为。

由索引模型根据 Issue 内容生成。