seedPackageRoot static method
Pre-seeds the package root so that files under root never trigger
directory-walking IO in _findPackageRoot.
Call this from standalone mode where the project root is known upfront.
Implementation
static void seedPackageRoot(String root) {
final normalized = root.replaceAll(r'\', '/');
_knownRoot = normalized;
_packageRootCache[normalized] = normalized;
// Also pre-load the project config to avoid IO during rule execution.
_cache.putIfAbsent(normalized, () => _loadProjectConfig(normalized));
}