fix(windows): capacity.cppm needs <stdlib.h>; keep bench child logs out of the measured tree

两个都是本轮自己引入的问题,CI 抓到的。

1. Windows 编译失败。`mcpp.platform.capacity` 的 Win32 分支用 malloc/free 处理
   GetLogicalProcessorInformationEx 的两段式调用,但 clang 不会从 windows.h
   拿到它们:

       error: no member named 'malloc' in the global namespace; did you mean '_alloca'?
       error: no type named 'free' in the global namespace

   补 <stdlib.h>。POSIX 侧同一类问题上一轮已经在 Darwin 上踩过一次
   (setenv/unsetenv 藏在 <_stdlib.h> 里),同样的修法。

2. bench 的 --project 模式把子进程日志写进了被测项目的根目录,于是
   `bench-child.log` 被 git add -A 顺手提交了进来。

   仅仅 gitignore 是治标:被测的那棵树在 --project 模式下就是用户的仓库,
   往里面丢文件本身才是问题。日志改为落在 work 目录下的 logs/,按
   引擎-场景命名;顺带 gitignore 兜底,并把已提交的那份删掉。

Author: speak-agent

.gitignore

@@ -35,4 +35,6 @@ compile_commands.json
 /bench/bench-work/
 bench-report.json
 bench/bench-report.json
+# --project mode writes the measured build's stdout/stderr next to the project
+bench-child.log
 .mcpp.toml.bench-backup

bench-child.log

@@ -71,6 +71,15 @@ public:
         fixture::Targets      targets;
     };
 
+    // Where child stdout/stderr is collected. Always under the work root, so it
+    // is disposable and never lands in the project being measured.
+    std::filesystem::path log_dir() const {
+        std::error_code ec;
+        auto d = opt_.work_root / "logs";
+        std::filesystem::create_directories(d, ec);
+        return d;
+    }
+
     Instance materialise(std::string_view engine, Variant variant) const {
         // PROJECT MODE. The tree already exists and belongs to someone; nothing
         // here may create or delete it. In particular the remove_tree below must
@@ -140,7 +149,12 @@ public:
         Job job;
         job.project_dir = inst.project_dir;
         job.build_dir   = inst.build_dir;
-        job.log_path    = inst.project_dir / "bench-child.log";
+        // The child log goes in the WORK directory, never inside the measured
+        // tree. In --project mode that tree is the user's repository, and a
+        // harness that drops files into it is one `git add -A` away from
+        // committing its own scratch (which is exactly what happened once).
+        job.log_path    = log_dir() / std::format("{}-{}.log", engine.name(),
+                                                  to_string(scenario));
         job.variant     = variant;
         job.profile     = std::string(profile);
         job.compiler    = std::string(compiler);

bench/src/runner.cppm

@@ -30,6 +30,7 @@ module;
 #define NOMINMAX
 #endif
 #include <windows.h>
+#include <stdlib.h>   // malloc / free — clang does not get these from windows.h
 #elif defined(__APPLE__)
 #include <sys/sysctl.h>
 #include <sys/types.h>