feat: require an llvm-family compiler, and name the compiler-runtime layer (#2)

* feat: declare the compiler-runtime layer and the compiler family it needs

这个包编译 729 个对象,其中 **498 个是 compiler-rt 的 builtins**、21 个是
libunwind 的、159 个 libcxx、51 个 libcxxabi。也就是说它最大的一块此前声明在
`mcpp:c++-abi` 名下 —— 而 `__udivti3` 及其同类是一个**纯 C 程序**需要的东西,
与 C++ 无关。

把 builtins 算作 C++ 运行时的一部分,等价于断言 C 程序不需要整数除法。
该断言已经产生过一次实测缺陷:一个交叉到 macOS 的 C 程序被问「有没有 C++
运行时」,答「没有」,链接行因而保留了编译器载荷自带的 libc++,
把一个 Linux 共享对象递给了 Mach-O 链接器。

    provides = [..., "mcpp:compiler-runtime=compiler-rt"]

── requires ──────────────────────────────────────────────

libc++ 的源码、尤其它的 std 模块源,由 clang 编译。递给 gcc 的实测结果:

    fatal error: __config: No such file or directory

该消息命名一个读者从未打开过的文件,以及一个 mcpp 从未作出的决定。
把需求写在包里,构建工具就能在编译任何东西之前拒绝这个组合,
并且能够指出族名,而不必把这条事实写进构建工具:

    requires = ["mcpp:compiler=llvm"]

实测(mcpp 2026.8.24.2):

    error: `openkal-llvm-runtime@0.1.1` requires the compiler to be `llvm`.
             compiler          gcc            (16.1.0, payload)
             required          llvm           (required by openkal-llvm-runtime@0.1.1)
           Select that compiler — yours outranks mcpp's own default:
               mcpp toolchain default llvm

── 兼容性 ────────────────────────────────────────────────

⚠️ 实测:**旧引擎(mcpp 2026.8.24.1)读带 `requires` 的清单构建成功** ——
TOML 侧忽略未知键。因此本次声明不要求使用者先升级。

`hosted-standard-library` 与 `mcpp:c++-abi=libc++` 两条拼写均保留。

engine: mcpp-community/mcpp#494

* 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

@@ -1,7 +1,7 @@
 [package]
 namespace   = "mcpplibs"
 name        = "openkal-llvm-runtime"
-version     = "0.1.1"
+version     = "0.1.2"
 description = "LLVM's C++ runtime libraries — libc++, libc++abi and libunwind — configured for openkal-musl rather than for a host C library."
 license     = "Apache-2.0"
 authors     = ["mcpplibs"]
@@ -20,6 +20,36 @@ repo        = "https://github.com/mcpplibs/openkal-llvm-runtime"
 # same layer; a newer engine reads the second and ignores the first.
 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.
+#
+# libc++'s sources, and its `std` module source above all, are compiled by
+# clang. Handing them to gcc fails inside libc++'s own headers, in a message
+# naming a file the reader has never opened. Stating the requirement here lets
+# the build tool refuse the combination before it compiles anything, and lets it
+# name the family without this fact being written into the build tool.
+requires = ["mcpp:compiler=llvm"]
+
 # Where this package's `std' module source is, and what it needs.
 #
 # The build tool otherwise asks the compiler (`-print-library-module-manifest-path'),