removeSection method
Remove a section from a memory file by heading.
Implementation
Future<MemoryFile?> removeSection(String path, String heading) async {
final existing = _files[path];
if (existing == null) return null;
final sections = MemoryParser.parse(existing.content);
final filtered = sections.where((s) => s.heading != heading).toList();
if (filtered.length == sections.length) return existing; // Not found.
final newContent = MemoryParser.build(filtered);
return save(path, newContent);
}