docs/22: which of the two new keys an older engine ignores, and which it refuses

`[kernel-abi]` is an unknown top-level table to an engine that predates it and
is ignored, so a package adopting it needs no floor move. `[c-abi.absent]` is a
new key inside a table mcpp does know, where an unrecognised key is a parse
error naming it --- the mechanism that keeps a misspelling from silently
disabling a declaration --- so a package adopting that one does.

Co-authored-by: Claude Code <noreply@anthropic.com>

Author: speak-agent

docs/22-target-side.md

@@ -599,6 +599,16 @@ list back when a link names one of the `link`-shaped entries, so
 `undefined reference to 'fork'` arrives with the sentence that says whether it
 is a defect or a limit of the environment this program was built for.
 
+**An engine that predates these keys.** `[kernel-abi]` is a top-level table an
+older mcpp does not know, and an unknown top-level table is ignored: a manifest
+carrying it loads unchanged everywhere it loaded before, and the absence of the
+resolution-time check is the behaviour that release already had. `[c-abi.absent]`
+is not the same case --- it is a new key inside a table mcpp does know, and an
+unrecognised key there is a parse error naming the key, which is what keeps a
+misspelling from silently disabling a declaration. A package adopting the second
+therefore requires an index floor at 2026.9.20.1; a package adopting the first
+does not.
+
 **Fingerprint.** The realised environment participates in the build's
 fingerprint (`compileFlags`, §92's field 7): two builds whose C library
 declares `lp64` and `llp64` compile the same source into objects whose

docs/zh/22-target-side.md

@@ -471,6 +471,12 @@ tcsetattr = { form = "accepted-no-effect", note = "openkal 不命名的那些字
 `undefined reference to 'fork'` 会带着那句说明它是缺陷还是这个程序所构建环境之限制的话
 一起到达。
 
+**早于这些键的引擎。** `[kernel-abi]` 是一个旧 mcpp 不认识的**顶层表**,而未知顶层表
+被忽略:带着它的清单在它此前能加载的每个地方照常加载,解析期检查的缺席正是那个版本本来
+就有的行为。`[c-abi.absent]` 不是同一种情形——它是 mcpp **认识的表里的一个新键**,而那里
+的未知键是一条点名该键的解析错误,这正是让拼错不会静默关掉一条声明的机制。因此采用后者
+的包需要把索引 floor 抬到 2026.9.20.1,采用前者的包不需要。
+
 **指纹。** 解析出的环境参与构建指纹(`compileFlags`,§92 的第 7 项):C 库声明 `lp64` 与
 `llp64` 的两次构建,从同一份源码编译出 `long` 宽度不同的目标文件,因此二者绝不共享输出目录,
 也不会复用对方产出的目标文件缓存。