v0.1.5 - 按 0.2.1 新契约移植：引用 chip 与图片都回来了

0.1.5 —— 按 DSH 0.2.1-rc.2 的新契约移植，并把 engines.dsh 从开口的 >=0.2.0-rc.1 收成 >=0.2.1-rc.2（旧区间会让它在 0.2.1 上照样安装、只静默降级）。

• 引用 chip 回来了：primitives.projectUserText 在 0.2.1 被删除（真机报 TypeError: primitives.projectUserText is not a function），投影改成调用宿主的文本工厂槽 conversation.user.text（facts.sessionLabels / facts.skillNames + interactive），chip 拆分逻辑不变。真机复验：控制台里那条降级告警不再出现（0 条），25/25 条用户气泡仍由本插件渲染。
• 图片附件回来了：座位 props 里的 renderMessageImages 已不存在，改调图片画廊工厂槽 conversation.images.content（附件插件自己注册的那个；它的 MessageImages 也只是这一步调用）。三条看起来更顺的路都被堵着，README 的「兼容性」一节记了实测报错原文。
• 性能：降级投影从「每帧算两次」改成只算一次（MarkdownBoundary 的 fallback 改成惰性 thunk，只有真失败时才构造）；toMarkdown() 加 64 条 LRU 缓存（流式期间同一条消息会被反复重排，而准备源码是 split/join + 逐行正则）；传给 MarkdownText 的 labels 对象按 t 记忆化（宿主自己的 ReasoningContent 正是这么做的，每次新建对象会让下游 memo 失效）。

v0.1.4 - one degradation function, not three copies

The degradation paths are one function now, not three copies.

"Composition failed", "the host projection threw" and "the last resort" each carried their own copy of
the fallback logic - one called the host projection, one inlined plain text - so three places had to be
kept in sync by hand. There is only degradedBody() now: it tries the host's own projection first
(which keeps reference chips and the rest of the host rendering), and falls back to plain text only if
the projection throws too. Content-first is part of the function's contract: plain text is the last
step, reached only when the host itself refuses to project.

New regression: projectUserText is called exactly once in the whole repo, the plain-text span is
constructed in exactly one place, and every degradation caller goes through the same function -
writing a second degradation copy turns the test red.