ci: 裸机那一步要的四个决定还没发布,所以从分支把工具建出来

这一步需要四个还没有任何已发布 mcpp 做出的决定,而它们都属于工具而不是本包:
import std 的门问「有没有包为这个目标提供标准库」而不是「目标是不是
freestanding」;包可以带自己的 std 模块源;图里有为目标编译的 C++ 运行时时
-fno-exceptions/-fno-rtti/-ffreestanding 要摘掉;以及展开表要打开 ——
编译器为这类目标默认关掉它,而一套不完整的展开表不会降级,它会让走栈停住。

都在 mcpp-community/mcpp#486 上。在它发布之前,这个 job 用和这里每一个依赖
同样的方式把工具拿过来 —— 从分支 —— 这样下面那条判据是**真的在被执行**,
而不是一句「在某台笔记本上验过一次」的注释。

⇒ #486 发布后,删掉这一步并抬高 MCPP_VERSION。

Author: speak-agent

.github/workflows/ci.yml

@@ -105,6 +105,38 @@ jobs:
           xlings install xim:qemu-riscv -y
           XLINGS_HOME="$HOME/.mcpp/registry" xlings install xim:qemu-riscv -y
 
+      # ⚠️ THE BUILD TOOL IS BUILT FROM A BRANCH, AND THAT IS TEMPORARY.
+      #
+      # This step needs four decisions that no released mcpp makes yet, each of
+      # which belongs to the tool rather than to this package:
+      #
+      #   `import std` is gated on whether a package PROVIDES a standard
+      #   library for the target rather than on whether the target is
+      #   freestanding; a package may carry its own std module source;
+      #   `-fno-exceptions` / `-fno-rtti` / `-ffreestanding` come off when the
+      #   graph supplies a C++ runtime built for the target; and unwind tables
+      #   go on, because the compiler turns them off for this kind of target
+      #   and a partial set of tables stops the walk rather than degrading it.
+      #
+      # They are on mcpp-community/mcpp#486. Until that is released this job
+      # builds the tool the same way every other dependency here is taken —
+      # from the branch — so the criterion below is actually enforced instead
+      # of being a comment saying it was verified once on a laptop.
+      #
+      # ⇒ When #486 ships, delete this step and raise MCPP_VERSION.
+      - name: The build tool, from the branch that has what this needs
+        if: matrix.toolchain == 'llvm@22.1.8'
+        run: |
+          set -euo pipefail
+          git clone --depth 1 -b feat/import-std-capability \
+            https://github.com/mcpp-community/mcpp "$RUNNER_TEMP/mcpp-src"
+          cd "$RUNNER_TEMP/mcpp-src"
+          mcpp build --release
+          BUILT=$(find target -type f -name mcpp -perm -u+x | head -1)
+          [ -n "$BUILT" ] || { echo "::error::mcpp did not build"; exit 1; }
+          echo "$(cd "$(dirname "$BUILT")" && pwd)" >> "$GITHUB_PATH"
+          "$BUILT" --version
+
       - name: The same program on bare metal, with exceptions
         if: matrix.toolchain == 'llvm@22.1.8'
         run: |