fix(bench): the same rewrite broke a third check — now there is a guard

`linux/clang/fixture` 84 ok / 6 failed:

    g++: error: unrecognized command-line option '--no-default-config'

clang 格子里 xmake 用了 **g++**,而描述里写的是 clang 的 flags。成因和我上一个
提交修的 `payload_toolchain` **一模一样**:main.cpp 把 `--compiler payload:clang`
改写成绝对路径后,`job.compiler == "clang"` 在**恰恰需要它为真的那些格子里**永远
为假,于是 `--toolchain=llvm` 没传,xmake 回落到默认编译器。

**我上次只修了一处,没有回头查还有谁在比同一个字符串。** 这是同一个改写第三次
咬人,每次在 review 里都读着像对的 —— 因为被比较的正是用户敲进去的那个词。

所以补的不只是那一行,还有 e2e 233 §7:**禁止任何引擎适配器按字面量比较
`job.compiler`**(`engine.cppm` 的 `resolve_cxx` 除外 —— 它是入口归一化,在改写
之前运行)。判据剔除注释行,两个方向都验过:改回字面量比较会红,还原后绿。

Author: speak-agent

bench/src/engines/xmake.cppm

@@ -153,7 +153,17 @@ public:
                 return platform::run(argv, job.buildfile_dir, job.log_path, job.timeout_s);
             }
         }
-        if (job.compiler == "clang") argv.push_back("--toolchain=llvm");
+        // Decided from the RESOLVED DRIVER, not from the literal string "clang"
+        // — main.cpp rewrites `--compiler payload:clang` into an absolute path
+        // before any engine sees it, so `job.compiler == "clang"` is false in
+        // exactly the cells that need this. xmake then fell back to g++ while
+        // the description carried clang's flags:
+        //     g++: error: unrecognized command-line option '--no-default-config'
+        // Six fixture cells in the linux/clang job. Same rewrite, same mistake
+        // as `payload_toolchain` above — which I fixed without checking whether
+        // anything else tested the same string.
+        if (job.compiler.find("clang") != std::string::npos)
+            argv.push_back("--toolchain=llvm");
         // The driver is pinned through CXX so every engine compiles with the
         // SAME binary; without it xmake resolves whatever `g++` means on this
         // host, and the comparison silently becomes compiler-vs-compiler.

tests/e2e/233_bench_matrix.sh

@@ -397,4 +397,41 @@ if bad:
 print(f"no cell schedules bazel against a ruleless package ({len(m['cells'])} cells)")
 PY
 
+# §7. No engine adapter may branch on the LITERAL compiler request.
+#
+# main.cpp resolves `--compiler payload:clang` into an absolute driver path
+# before any engine sees it, so `job.compiler == "clang"` is false in exactly
+# the cells that mean clang. That rewrite has now broken three separate checks:
+#   * `payload_toolchain` — --toolchain=mcpp-* was never passed at all
+#   * `--toolchain=llvm`  — xmake fell back to g++ with clang's flags:
+#                           `g++: unrecognized command-line option
+#                           '--no-default-config'`, six fixture cells red
+#   * (the same shape would hit any new one written the same way)
+#
+# Each time it looked correct in review, because the string being compared is
+# the string the user typed. Adapters must key off the RESOLVED PATH instead.
+python3 - "$ROOT" <<'PY' || exit 1
+import pathlib, re, sys
+root = pathlib.Path(sys.argv[1]) / "bench/src/engines"
+bad = []
+for f in root.glob("*.cppm"):
+    # engine.cppm's resolve_cxx() is the NORMALISER — comparing there is how a
+    # bare `gcc` becomes `g++`, and it runs before any rewrite. Everything else
+    # sees the resolved path.
+    if f.name == "engine.cppm":
+        continue
+    for n, line in enumerate(f.read_text().splitlines(), 1):
+        code = line.split("//", 1)[0]
+        if re.search(r'compiler\s*==\s*"(clang|gcc)"', code):
+            bad.append(f"{f.name}:{n}: {line.strip()[:90]}")
+if bad:
+    print("FAIL: an engine adapter compares job.compiler to a literal:")
+    for b in bad:
+        print("  " + b)
+    print("  main.cpp rewrites payload:* into a path first, so that test never fires.")
+    print("  Key off the resolved driver path (see payload_toolchain).")
+    sys.exit(1)
+print("no engine adapter branches on the literal compiler request")
+PY
+
 echo "bench matrix OK"