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.go

@@ -0,0 +1,61 @@
+package testrun
+
+import (
+	"context"
+	"fmt"
+	"os/exec"
+)
+
+// Preflight runs platform-specific host checks before sidecar/driver setup.
+// On failure it returns a wrapped error pointing the user at the matching
+// `sanderling doctor --platform=<p>` command. Web returns nil (no host
+// prerequisites beyond a working chromium, which the driver will surface
+// itself if missing).
+func Preflight(ctx context.Context, platform string) error {
+	check := preflightCheck
+	return runPreflight(ctx, platform, check)
+}
+
+type preflightFunc func(name string) error
+
+func preflightCheck(name string) error {
+	if _, err := exec.LookPath(name); err != nil {
+		return fmt.Errorf("%s not found on PATH: %w", name, err)
+	}
+	return nil
+}
+
+func runPreflight(ctx context.Context, platform string, check preflightFunc) error {
+	if err := ctx.Err(); err != nil {
+		return err
+	}
+	switch platform {
+	case "web":
+		return nil
+	case "android":
+		if err := check("adb"); err != nil {
+			return preflightFailure("android", err)
+		}
+		if err := check("java"); err != nil {
+			return preflightFailure("android", err)
+		}
+		return nil
+	case "ios":
+		if err := check("xcrun"); err != nil {
+			return preflightFailure("ios", err)
+		}
+		if err := check("java"); err != nil {
+			return preflightFailure("ios", err)
+		}
+		return nil
+	default:
+		return fmt.Errorf("preflight: unknown platform %q", platform)
+	}
+}
+
+func preflightFailure(platform string, cause error) error {
+	return fmt.Errorf(
+		"preflight: %w\nrun `sanderling doctor --platform=%s` for full host-readiness checks",
+		cause, platform,
+	)
+}