feat(testrun): add Preflight() before sidecar/driver setup

Mobile platforms get a friendly install hint pointing at
`sanderling doctor --platform=<p>` instead of `fork/exec java: not found`.
Web is a no-op.

internal/testrun/preflight_test.go

@@ -0,0 +1,82 @@
+package testrun
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+)
+
+func TestPreflight_WebSkips(t *testing.T) {
+	called := 0
+	check := func(name string) error {
+		called++
+		return nil
+	}
+	if err := runPreflight(context.Background(), "web", check); err != nil {
+		t.Fatalf("web preflight should be no-op, got %v", err)
+	}
+	if called != 0 {
+		t.Errorf("web preflight ran %d binary checks; expected 0", called)
+	}
+}
+
+func TestPreflight_AndroidNeedsAdbAndJava(t *testing.T) {
+	cases := []struct {
+		name      string
+		missing   string
+		wantInErr string
+	}{
+		{name: "missing adb", missing: "adb", wantInErr: "adb"},
+		{name: "missing java", missing: "java", wantInErr: "java"},
+	}
+	for _, testCase := range cases {
+		t.Run(testCase.name, func(t *testing.T) {
+			check := func(name string) error {
+				if name == testCase.missing {
+					return errors.New(name + " not found")
+				}
+				return nil
+			}
+			err := runPreflight(context.Background(), "android", check)
+			if err == nil || !strings.Contains(err.Error(), testCase.wantInErr) {
+				t.Fatalf("expected error mentioning %q, got %v", testCase.wantInErr, err)
+			}
+			if !strings.Contains(err.Error(), "sanderling doctor --platform=android") {
+				t.Errorf("error missing doctor hint: %v", err)
+			}
+		})
+	}
+}
+
+func TestPreflight_iOSNeedsXcrunAndJava(t *testing.T) {
+	check := func(name string) error {
+		if name == "xcrun" {
+			return errors.New("xcrun not found")
+		}
+		return nil
+	}
+	err := runPreflight(context.Background(), "ios", check)
+	if err == nil || !strings.Contains(err.Error(), "xcrun") {
+		t.Fatalf("expected xcrun error, got %v", err)
+	}
+	if !strings.Contains(err.Error(), "sanderling doctor --platform=ios") {
+		t.Errorf("error missing doctor hint: %v", err)
+	}
+}
+
+func TestPreflight_AllOK(t *testing.T) {
+	check := func(name string) error { return nil }
+	for _, platform := range []string{"web", "android", "ios"} {
+		if err := runPreflight(context.Background(), platform, check); err != nil {
+			t.Errorf("%s: unexpected error %v", platform, err)
+		}
+	}
+}
+
+func TestPreflight_UnknownPlatform(t *testing.T) {
+	check := func(string) error { return nil }
+	if err := runPreflight(context.Background(), "fuchsia", check); err == nil {
+		t.Error("expected error for unknown platform")
+	}
+}