feat(cli): add --max-steps for step-bounded runs

runner.Options.MaxSteps already worked but was unreachable from the command
line. A step budget is what makes two generators comparable: one making a
model call per step and one drawing from a PRNG are not comparable per second.

Claude-Session: https://claude.ai/code/session_01A5KmftdEJ49A9z5mF5ESrX

cmd/sanderling/main.go

@@ -27,6 +27,7 @@ type testOptions struct {
 	iosAppPath     string
 	androidAppPath string
 	duration       time.Duration
+	maxSteps       int
 	seed           int64
 	output         string
 	clearData      bool
@@ -60,6 +61,7 @@ func parseTestArgs(args []string, stderr io.Writer) (testOptions, error) {
 	flagSet.StringVar(&options.iosAppPath, "ios-app-path", "", "path to the .app bundle for iOS clear-state reinstall (simulator: simctl; device: devicectl)")
 	flagSet.StringVar(&options.androidAppPath, "android-app-path", "", "path to the .apk for Android clear-state reinstall; required to reset apps on OEM builds that deny `pm clear`")
 	flagSet.DurationVar(&options.duration, "duration", 5*time.Minute, "total test duration")
+	flagSet.IntVar(&options.maxSteps, "max-steps", 0, "stop after this many steps (0 = no cap; the duration deadline governs). A step budget is what makes two generators comparable, since one making a model call per step and one drawing from a PRNG are not comparable per second")
 	flagSet.Int64Var(&options.seed, "seed", 0, "RNG seed (0 = random)")
 	flagSet.StringVar(&options.output, "output", "./runs", "output directory for traces")
 	flagSet.BoolVar(&options.clearData, "clear-data", true, "clear app data before launching so each run starts from a fresh install; pass --clear-data=false to resume prior state")
@@ -78,6 +80,9 @@ func parseTestArgs(args []string, stderr io.Writer) (testOptions, error) {
 	default:
 		return testOptions{}, fmt.Errorf("unsupported platform: %q (android, ios, web)", options.platform)
 	}
+	if options.maxSteps < 0 {
+		return testOptions{}, fmt.Errorf("--max-steps must not be negative: %d", options.maxSteps)
+	}
 	switch options.generator {
 	case "seeded", "llm":
 	default:

cmd/sanderling/main_test.go

@@ -259,3 +259,41 @@ func TestRun_TestSubcommand_PipelineErrors(t *testing.T) {
 		t.Errorf("expected bundle-resolution error for the missing spec, got %v", err)
 	}
 }
+
+func TestParseTestArgs_MaxStepsDefaultsToUncapped(t *testing.T) {
+	options, err := parseTestArgs([]string{
+		"--spec", "s.ts",
+		"--bundle-id", "com.example",
+	}, io.Discard)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if options.maxSteps != 0 {
+		t.Errorf("maxSteps default: got %d, want 0", options.maxSteps)
+	}
+}
+
+func TestParseTestArgs_MaxSteps(t *testing.T) {
+	options, err := parseTestArgs([]string{
+		"--spec", "s.ts",
+		"--bundle-id", "com.example",
+		"--max-steps", "300",
+	}, io.Discard)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if options.maxSteps != 300 {
+		t.Errorf("maxSteps: got %d, want 300", options.maxSteps)
+	}
+}
+
+func TestParseTestArgs_RejectsNegativeMaxSteps(t *testing.T) {
+	_, err := parseTestArgs([]string{
+		"--spec", "s.ts",
+		"--bundle-id", "com.example",
+		"--max-steps", "-1",
+	}, io.Discard)
+	if err == nil {
+		t.Fatal("expected an error for a negative --max-steps")
+	}
+}

cmd/sanderling/test_run.go

@@ -18,6 +18,7 @@ func runTestPipeline(ctx context.Context, options testOptions, stdout io.Writer)
 		IosAppPath:     options.iosAppPath,
 		AndroidAppPath: options.androidAppPath,
 		Duration:       options.duration,
+		MaxSteps:       options.maxSteps,
 		Seed:           options.seed,
 		Output:         options.output,
 		ClearData:      options.clearData,

internal/testrun/testrun.go

@@ -31,6 +31,7 @@ type Options struct {
 	IosAppPath     string
 	AndroidAppPath string
 	Duration       time.Duration
+	MaxSteps       int
 	Seed           int64
 	Output         string
 	ClearData      bool
@@ -159,9 +160,14 @@ func Execute(ctx context.Context, options Options, stdout io.Writer) error {
 	}()
 	fmt.Fprintf(stdout, "trace dir: %s\n", runDirectory)
 
+	if options.MaxSteps > 0 {
+		fmt.Fprintf(stdout, "running for %s or %d steps, whichever comes first (seed=%d)\n", options.Duration, options.MaxSteps, seed)
+	} else {
 		fmt.Fprintf(stdout, "running for %s (seed=%d)\n", options.Duration, seed)
+	}
 	summary, err := runner.Run(ctx, runner.Options{
 		Duration:    options.Duration,
+		MaxSteps:    options.MaxSteps,
 		IdleTimeout: 1 * time.Second,
 		BundleID:    options.BundleID,
 		Driver:      activeDriver,