fix(bench): a bracketed engine option must survive the --engines list split

`--engines` 用逗号分隔 spec,而 bracket 选项之间也用逗号 —— 于是
`mcpp[a=1,b=2]=/path` 会先被列表切成 `mcpp[a=1` 和 `b=2]=/path` 两半,
`make_engine` 收到的是碎片,报「unknown engine」,而 spec 本身完全合法。

分隔符相同,所以**得由切列表的那一侧认识方括号**。split() 加了一个深度计数;
对 `--variants` / `--scenarios` / `--allow-failed` 没有影响(它们不含方括号,
计数永远是 0)。

顺带把 registry 里那个多余的 `{}` 块拍平。

Author: speak-agent

bench/src/main.cpp

@@ -73,15 +73,28 @@ bool listed(const std::vector<std::string>& names, std::string_view engine) {
     });
 }
 
+// Splits a comma-separated list, IGNORING commas inside `[...]`.
+//
+// An engine spec may carry bracketed options (`mcpp[schedule=on]=/path`), and a
+// second option would be separated by a comma — which this function would
+// otherwise cut in half, handing `make_engine` the fragments `mcpp[a=1` and
+// `b=2]=/path` and reporting "unknown engine" for a perfectly valid spec. The
+// list separator and the option separator are the same character, so the list
+// splitter is the one that has to know about the brackets.
+//
+// Harmless for every other caller: `--variants`, `--scenarios` and
+// `--allow-failed` contain no brackets, so the depth counter never leaves zero.
 std::vector<std::string> split(std::string_view s, char sep = ',') {
     std::vector<std::string> parts;
-    std::size_t start = 0;
-    while (start <= s.size()) {
-        const auto pos = s.find(sep, start);
-        const auto end = (pos == std::string_view::npos) ? s.size() : pos;
-        if (end > start) parts.emplace_back(s.substr(start, end - start));
-        if (pos == std::string_view::npos) break;
-        start = pos + 1;
+    std::size_t start = 0, depth = 0;
+    for (std::size_t i = 0; i <= s.size(); ++i) {
+        if (i < s.size()) {
+            if (s[i] == '[') { ++depth; continue; }
+            if (s[i] == ']') { if (depth) --depth; continue; }
+            if (s[i] != sep || depth) continue;
+        }
+        if (i > start) parts.emplace_back(s.substr(start, i - start));
+        start = i + 1;
     }
     return parts;
 }

bench/src/registry.cppm

@@ -91,18 +91,16 @@ inline std::unique_ptr<engines::Engine> make_engine(std::string_view spec) {
         program = anchor_program(std::string(spec.substr(eq + 1)));
     }
 
-    {
-        for (std::size_t at = 0; at <= opts.size();) {
-            const auto end  = std::min(opts.find(',', at), opts.size());
-            const auto item = std::string_view(opts).substr(at, end - at);
-            at = end + 1;
-            if (item.empty()) continue;
-            const auto sep = item.find('=');
-            if (sep == std::string_view::npos) return nullptr;
-            auto mapped = engine_option(name, item.substr(0, sep), item.substr(sep + 1));
-            if (!mapped) return nullptr;                  // unknown: reject loudly
-            env.emplace(std::move(mapped->first), std::move(mapped->second));
-        }
+    for (std::size_t at = 0; at <= opts.size();) {
+        const auto end  = std::min(opts.find(',', at), opts.size());
+        const auto item = std::string_view(opts).substr(at, end - at);
+        at = end + 1;
+        if (item.empty()) continue;
+        const auto sep = item.find('=');
+        if (sep == std::string_view::npos) return nullptr;
+        auto mapped = engine_option(name, item.substr(0, sep), item.substr(sep + 1));
+        if (!mapped) return nullptr;                  // unknown: reject loudly
+        env.emplace(std::move(mapped->first), std::move(mapped->second));
     }
 
     if (name == "mcpp")