docs: the gate on the layer declaration is the index, not this package's floor (#3)

推迟 `provides = ["mcpp:compiler-runtime=…"]` 的注释把判据写成了
「本包的 floor 移到 2026.8.24.2」。判据不是这个 —— 读这份文件的人
手上有什么,由**索引服务哪个版本**决定,和本包声明的 floor 无关。

2026-08-24 实测,依赖各声明一个键,构建工具取索引 `latest` 指向的版本:

    requires = ["mcpp:compiler=llvm"]        2026.8.24.1   exit 0,静默忽略
    provides = ["mcpp:compiler-runtime=…"]   2026.8.24.1   exit 2,整份拒绝

⭐ 于是两个键的答案是分开的:`requires` 今天就能发,层声明不能。
而 24.2 引入的那份宽容对现实中的任何用户都还不存在 —— **24.2 从未进索引**,
索引的 latest 停在 24.1。「新版本支持了」和「用户拿得到」是两件事。

开门条件改写为:索引的 `latest` 到达 2026.8.24.2 或以上。

Author: Sunrisepeak

mcpp.toml

@@ -38,8 +38,19 @@ provides = ["hosted-standard-library", "mcpp:c++-abi=libc++"]
 #
 # ⚠️ That refusal is itself fixed in 2026.8.24.2 — an unknown layer name in a
 # DEPENDENCY's manifest is now ignored with a warning rather than refused — but
-# the fix cannot apply retroactively to tools already released. The declaration
-# is therefore added once this package's floor moves to 2026.8.24.2.
+# the fix cannot apply retroactively to tools already released.
+#
+# ⚠️ AND THE CRITERION IS NOT THIS PACKAGE'S FLOOR. It is what the index serves,
+# because that is what a reader of this file actually has. Measured 2026-08-24
+# against the two keys, with a dependency declaring each and a build tool taken
+# from the version the index named `latest`:
+#
+#   requires = ["mcpp:compiler=llvm"]              2026.8.24.1  exit 0, ignored
+#   provides = ["mcpp:compiler-runtime=…"]         2026.8.24.1  exit 2, refused
+#
+# So `requires` is safe to publish today and the layer declaration is not. The
+# gate on adding it is that the index's `latest` be at 2026.8.24.2 or above —
+# not that a newer mcpp exists, and not that this package's floor was raised.
 
 # What this package needs of the layer it does not supply.
 #