feat: require an llvm-family compiler, and name the compiler-runtime layer

⚠️ 实测:该层名在 2026.8.24.2 之前的每一个已发布构建工具上都让整份清单
加载失败 —— 不是「该层被忽略」,是这个包用不了:

    error: dependency 'openkal-llvm-runtime': mcpp.toml: error:
           `provides = ["mcpp:compiler-runtime=compiler-rt"]` names no
           capability mcpp knows.

那条拒绝本身已在 mcpp 2026.8.24.2 修掉(依赖的清单里出现未知层名改为警告并
忽略,根工程自己的仍然报错),但**修复不能追溯到已经发布出去的工具**。
本仓 CI 的 MCPP_VERSION 目前是 2026.8.19.4。

`requires = ["mcpp:compiler=llvm"]` 保留 —— 实测旧引擎忽略未知的 [package] 键,
因此这一半今天就能发。声明留在注释里,连同它的依据与解除条件。

engine: mcpp-community/mcpp#494

Author: Sunrisepeak

mcpp.toml

@@ -18,18 +18,28 @@ repo        = "https://github.com/mcpplibs/openkal-llvm-runtime"
 # The older spelling is kept beside the current one so that an engine
 # predating the layer vocabulary still recognises this package. Both name the
 # same layer; a newer engine reads the second and ignores the first.
-provides = [
-  "hosted-standard-library",
-  "mcpp:c++-abi=libc++",
-  # ⭐ AND THE COMPILER RUNTIME, WHICH IS THE LARGER HALF OF THIS PACKAGE.
-  #
-  # Measured: of the 729 objects this package builds, 498 are compiler-rt's
-  # builtins and 21 are libunwind's. Those are what a C PROGRAM needs —
-  # `__udivti3` and its relatives have nothing to do with C++ — and declaring
-  # them under the C++ layer said this package supplied something it does not
-  # and concealed something it does.
-  "mcpp:compiler-runtime=compiler-rt",
-]
+provides = ["hosted-standard-library", "mcpp:c++-abi=libc++"]
+
+# ⚠️ THE COMPILER RUNTIME IS THE LARGER HALF OF THIS PACKAGE AND IS NOT YET
+# DECLARED, BECAUSE DECLARING IT WOULD MAKE THIS MANIFEST UNREADABLE TO EVERY
+# RELEASED BUILD TOOL.
+#
+# Measured: of the 729 objects built here, 498 are compiler-rt's builtins and 21
+# are libunwind's. Those are what a C PROGRAM needs — `__udivti3` and its
+# relatives have nothing to do with C++ — so declaring them under the C++ layer
+# says this package supplies something it does not and conceals something it
+# does.
+#
+# The layer is named `mcpp:compiler-runtime` from mcpp 2026.8.24.2, and a build
+# tool released before that reports:
+#
+#   error: `provides = ["mcpp:compiler-runtime=compiler-rt"]` names no
+#          capability mcpp knows.
+#
+# ⚠️ 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.
 
 # What this package needs of the layer it does not supply.
 #