Add deterministic C++ reference adapter

tests/fixtures/reference-cpp/compile_commands.json

@@ -0,0 +1,19 @@
+[
+  {
+    "arguments": [
+      "c++",
+      "-std=c++20",
+      "-Iinclude",
+      "-c",
+      "src/main.cpp"
+    ],
+    "directory": ".",
+    "file": "src/main.cpp"
+  },
+  {
+    "command": "c++ -std=c++20 -Iinclude -c src/worker.cpp",
+    "directory": ".",
+    "file": "src/worker.cpp",
+    "output": "worker.o"
+  }
+]

tests/fixtures/reference-cpp/include/config.hpp

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+#pragma once
+
+#include "detail.hpp"
+
+namespace demo {
+
+struct Config {
+    int limit;
+};
+
+inline int effective_limit(const Config& config) {
+    return normalize(config.limit);
+}
+
+}  // namespace demo

tests/fixtures/reference-cpp/include/detail.hpp

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+#pragma once
+
+namespace demo {
+
+inline int normalize(int value) {
+    if (value < 0) {
+        return 0;
+    }
+    return value;
+}
+
+}  // namespace demo

tests/fixtures/reference-cpp/src/main.cpp

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+#include "../include/config.hpp"
+
+namespace demo {
+
+int execute(int value) {
+    Config config{value};
+    if (value == 0) {
+        return effective_limit(config);
+    }
+    return normalize(value);
+}
+
+}  // namespace demo
+
+int main() {
+    return demo::execute(3);
+}

tests/fixtures/reference-cpp/src/worker.cpp

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+#include "../include/config.hpp"
+
+namespace demo {
+
+class Worker {
+public:
+    int run(int value) {
+        for (int attempt = 0; attempt < 2; ++attempt) {
+            if (value > attempt) {
+                return effective_limit(Config{value});
+            }
+        }
+        return 0;
+    }
+};
+
+}  // namespace demo