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

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"]
@@ -18,7 +18,27 @@ 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++"]
+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",
+]
+
+# 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.
 #