VariableReference.Clone() throws on an empty name, crashing C/C++/Rust/Go generation (and the editor preview) for any class with a property #161

What's wrong

• VariableReference.Clone() builds its copy with new(Name) (Coder/Ast/VariableReference.cs:40), and that constructor throws ArgumentException on an empty or whitespace name (L20).
• Empty names are easy to produce legitimately:
  • The parameterless constructor sets Name = string.Empty.
  • new AssignmentStatement() sets Target = new VariableReference() (Coder/Ast/AssignmentStatement.cs:49).
  • YamlDeserializer.DeserializeVariableReference sets Name = ReadString(...) ?? "" (Coder/Serialization/YamlDeserializer.cs ~L1298-1310).
• StandardLanguageGenerator.Separated clones the whole class whenever it has a property, and also clones property bodies (Coder/Languages/StandardLanguageGenerator.cs ~L494, ~L558, ~L571).
• AstSchema.IsUnfilled (Coder.Graph/AstSchema.cs ~L631) only flags empty string literals, so Validate() reports nothing.

Reproduction

Deserialize a class with one propertyDeclaration and a method whose body contains an assignmentStatement that has only a value: and no target. A new assignment node dropped in the editor is in the same state. C#, Python and JavaScript generate fine. C++, C, Rust and Go throw ArgumentException: Name cannot be null or whitespace.

In the editor, CoderEditorApp.Regenerate (Coder.Editor/CoderEditorApp.cs:637-655) runs from the preview pane every frame but catches only NotSupportedException. Its own remarks say an exception thrown in a frame takes the application down. Choosing one of those four preview languages with a half-filled assignment therefore crashes the editor.

Related but separate: #112 hits the same constructor through MemberInitialiser fallbacks. This one comes through Clone().

Suggested fix / acceptance criteria

• Make Clone() use the parameterless constructor and copy Name, so cloning never validates.
• Optionally, have Validate() flag an empty VariableReference, and/or have Regenerate also catch ArgumentException/InvalidOperationException and show the message in the preview.
• Add a reflective test asserting that Clone() succeeds for every AST node type built with its parameterless constructor.