fix: context reuse benchmark uses temp directory

- Replace hardcoded testdata path with b.TempDir()
- Add createSmallDataset helper for self-contained benchmarks
- Fixes FAIL: BenchmarkContextReuse / BenchmarkSequentialHashes

internal/worker/native_bridge_libs.go

@@ -77,3 +77,9 @@ func ScanArtifactsNative(runDir string) (*manifest.Artifacts, error) {
 func ExtractTarGzNative(archivePath, dstDir string) error {
 	return ExtractTarGz(archivePath, dstDir)
 }
+
+// DirOverallSHA256HexNative exports the native hash implementation for benchmarks.
+// This allows explicit native library usage when -tags native_libs is enabled.
+func DirOverallSHA256HexNative(root string) (string, error) {
+	return dirOverallSHA256HexNative(root)
+}

tests/benchmarks/context_reuse_bench_test.go

@@ -1,16 +1,37 @@
 package benchmarks
 
 import (
+	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/jfraeys/fetch_ml/internal/worker"
 )
 
+// createSmallDataset creates a temporary small dataset for benchmarking
+func createSmallDataset(b *testing.B) string {
+	tmpDir := b.TempDir()
+	// Create 10 small files (100KB each) to emphasize context overhead
+	data := make([]byte, 100*1024)
+	for i := range data {
+		data[i] = byte(i % 256)
+	}
+	for i := range 10 {
+		path := filepath.Join(tmpDir, "data", string(rune('a'+i%26)), "chunk.bin")
+		if err := os.MkdirAll(filepath.Dir(path), 0750); err != nil {
+			b.Fatal(err)
+		}
+		if err := os.WriteFile(path, data, 0640); err != nil {
+			b.Fatal(err)
+		}
+	}
+	return tmpDir
+}
+
 // BenchmarkContextReuse measures overhead of repeated hash operations
 // This verifies the 5-20ms savings from context reuse in native_bridge_libs.go
 func BenchmarkContextReuse(b *testing.B) {
-	// Small test directory to emphasize context overhead vs I/O
-	testDir := "./testdata/small_dataset"
+	testDir := createSmallDataset(b)
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
@@ -25,7 +46,7 @@ func BenchmarkContextReuse(b *testing.B) {
 // With context reuse: ~8ms per hash
 // Without context reuse: ~17ms per hash (9ms overhead)
 func BenchmarkSequentialHashes(b *testing.B) {
-	testDir := "./testdata/small_dataset"
+	testDir := createSmallDataset(b)
 
 	b.ReportAllocs()
 	b.ResetTimer()