refactor: use b.Run for sub-benchmarks and b.Loop for iteration

Replace individual benchmark functions with a single BenchmarkSamples function.
Use b.Run to create sub-benchmarks for each sample file.
Update all benchmarks to use b.Loop instead of manual b.N iteration.

cmd/lambda/benchmark_test.go

@@ -0,0 +1,70 @@
+package main
+
+import (
+	"os"
+	"testing"
+
+	"git.maximhutz.com/max/lambda/internal/config"
+	"git.maximhutz.com/max/lambda/internal/engine"
+	"git.maximhutz.com/max/lambda/pkg/convert"
+	"git.maximhutz.com/max/lambda/pkg/lambda"
+	"git.maximhutz.com/max/lambda/pkg/saccharine"
+)
+
+// Helper function to run a single sample through the lambda interpreter.
+func runSample(samplePath string) error {
+	// Read the sample file.
+	input, err := os.ReadFile(samplePath)
+	if err != nil {
+		return err
+	}
+
+	// Parse code into syntax tree.
+	ast, err := saccharine.Parse(string(input))
+	if err != nil {
+		return err
+	}
+
+	// Compile expression to lambda calculus.
+	compiled := convert.SaccharineToLambda(ast)
+
+	// Create minimal config for benchmarking.
+	cfg := &config.Config{
+		Source:      config.StringSource{Data: ""},
+		Destination: config.StdoutDestination{},
+		Profile:     "",
+		Explanation: false,
+		Statistics:  false,
+		Verbose:     false,
+	}
+
+	// Create and run the engine.
+	process := engine.New(cfg, &compiled)
+	process.Run()
+
+	// Get final result (to ensure it's not optimized away).
+	_ = lambda.Stringify(compiled)
+
+	return nil
+}
+
+// Benchmark all samples using sub-benchmarks.
+func BenchmarkSamples(b *testing.B) {
+	samples := map[string]string{
+		"Church":     "../../samples/church.txt",
+		"Fast":       "../../samples/fast.txt",
+		"Saccharine": "../../samples/saccharine.txt",
+		"Simple":     "../../samples/simple.txt",
+		"Thunk":      "../../samples/thunk.txt",
+	}
+
+	for name, path := range samples {
+		b.Run(name, func(b *testing.B) {
+			for b.Loop() {
+				if err := runSample(path); err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+	}
+}