Along/across-stroke gradients

There are two directions:

- Along the stroke

[See this](https://dev.to/uuuuuulala/gradient-along-svg-path-with-gsap-kl4) for info about how to approximate this. We can also just split up the stroke into a lot of pieces with solids colors, or with linear gradients aiming to go from the start to end, using greater subdivision where the start and end planes bend more quickly to aim for them to be close to parallel so the linear gradient works.


- Across the stroke

Repeat the stroke many times with increasing thickness. Use the technique described in #2268 for not mirroring at the centerline.


Regardless of method or direction, masking is likely necessary to handle end caps properly.

Start by reviewing the two proposed approaches in this issue and the technique described in #2268. Decide how along-stroke and across-stroke gradients should be represented, including subdivision, repeated strokes, and masking for end caps. Done means both directions render correctly without centerline mirroring or end-cap artifacts.

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