A method body can't portably refer to its own instance: field reads and sibling calls need this./self./self-> text in VariableReference, so no such body translates across targets #141

What's wrong

The AST has no node for "the current instance" and no member-access expression. The only way for a method body to read a field, or call another method on its own object, is to put target-language text into VariableReference. The test suite does this itself, with new VariableReference("self.x") and new CallExpression(new VariableReference("self"), "Shift"). CompiledExemplar records the limitation:

a body that does says self->x in C and self.x in the other two, which is a difference no shared declaration can hold.

The generators already know each language's receiver. Python writes self into the signature, C and Go name the receiver self, and C#/JS write this. for member initialisers. Bodies just have no way to ask for it.

Failure scenario

Take a class Counter with a field count and a method add(n) whose body is count += helper(n), where helper is another instance method. Using a bare VariableReference("count") and CallExpression("helper"), every generator emits count += helper(n):

• Python: UnboundLocalError on count, NameError on helper.
• C: count and helper are undeclared, since the function is Counter_helper(self, n).
• Rust, Go: unresolved names.

Changing the text to self.count fixes Python, Rust and Go, but breaks C# (no self), C++ (this->count), JS (this.count) and C (self->count). No single AST compiles in all seven targets, which undermines the premise of a language-agnostic AST. It is the same kind of gap that #55 closed for calls.

Suggested fix / acceptance criteria

• Add a ThisExpression (or SelfReference) node, and a MemberAccessExpression(Receiver, MemberName) for field reads and writes. CallExpression.Receiver already covers calls.
• Each generator spells them in its own way:
  • C# and JS: this.x
  • C++: this->x
  • Python, Rust and Go: self.x
  • C: self->x, with instance calls as Type_method(self, …)
• The new nodes get the same support as existing ones: YAML serializer and deserializer, AstSchema/AstFields/AstNodeCatalog, and the node-exhaustiveness test.
• Rewrite CompiledExemplar so its member-reading group uses the new nodes and is shared with C.