• Breaking Change: sdkPath now returns String? (returning null if no
  valid Dart SDK can be located).
• Support robust SDK discovery for AOT-compiled tools (dart install,
  dart build cli) probing Platform.resolvedExecutable, DART_ROOT,
  DART_SDK, system PATH (with Flutter wrapper layout awareness), and
  FLUTTER_ROOT.
• Add dartExecutable getter (String?) to locate the dart binary.
• Add isValidSdkPath(String candidatePath) validation helper.