add derivation parser benchmark

the current identified bottlenecks are parseString in derivations.cc and dirOf (because of std::filessystem creation).

src/libstore-tests/derivation-parser-bench.cc

@@ -0,0 +1,45 @@
+#include <benchmark/benchmark.h>
+#include "nix/store/derivations.hh"
+#include "nix/store/store-api.hh"
+#include "nix/util/experimental-features.hh"
+#include "nix/store/store-open.hh"
+#include "nix/store/globals.hh"
+#include <fstream>
+#include <sstream>
+
+using namespace nix;
+
+// Benchmark parsing real derivation files
+static void BM_ParseRealDerivationFile(benchmark::State & state, const std::string & filename)
+{
+    // Read the file once
+    std::ifstream file(filename);
+    std::stringstream buffer;
+    buffer << file.rdbuf();
+    std::string content = buffer.str();
+
+    auto store = openStore("dummy://");
+    ExperimentalFeatureSettings xpSettings;
+
+    for (auto _ : state) {
+        auto drv = parseDerivation(*store, std::string(content), "test", xpSettings);
+        benchmark::DoNotOptimize(drv);
+    }
+    state.SetBytesProcessed(state.iterations() * content.size());
+}
+
+// Register benchmarks for actual test derivation files if they exist
+BENCHMARK_CAPTURE(BM_ParseRealDerivationFile, hello, std::string(NIX_UNIT_TEST_DATA) + "/derivation/hello.drv");
+BENCHMARK_CAPTURE(BM_ParseRealDerivationFile, firefox, std::string(NIX_UNIT_TEST_DATA) + "/derivation/firefox.drv");
+
+// Custom main to initialize Nix before running benchmarks
+int main(int argc, char ** argv)
+{
+    // Initialize libstore
+    nix::initLibStore(false);
+
+    // Initialize and run benchmarks
+    ::benchmark::Initialize(&argc, argv);
+    ::benchmark::RunSpecifiedBenchmarks();
+    return 0;
+}