Constructing a declared type with member initialisers gives wrong fields in JavaScript (new Point({x:0,y:0}) sets x to the object) and a TypeError in Python #335

What's wrong

For a ConstructionExpression with MemberInitialiser arguments, the JavaScript generator emits new T({ x: …, y: … }) (JavaScriptGenerator.cs L233-L299). Its doc comment calls this "the options-object convention". The classes the same generator writes don't follow that convention:

• Class with a constructor: the constructor takes positional parameters (constructor(x, y = 0)), so the whole options object lands in the first parameter.
• Class with no constructor: the argument is ignored, so every field stays undefined.

Python emits T(x=…, y=…) (PythonGenerator.cs L354-L431). That works only when the class happens to have a constructor whose parameters have those names. A class with fields and no constructor has no __init__ that accepts keywords, so the call raises TypeError.

Reproduction (HEAD 667637c)

1. Repo exemplar CompiledExemplar.Point() (constructor Point(x, y = 0)), with zero() and OriginTable(). Both results are silently wrong in JavaScript:
   • Point.zero() gives {"x":{"x":0,"y":0},"y":0}
   • ORIGINS[1] is {"x":{"x":1,"y":1},"y":0}
2. Class with no constructor: class Vec { public int x; public int y; } and Vec make() { return Vec{ x = 1, y = 2 }; }.
   • JavaScript: make().x is undefined.
   • Python: make() raises TypeError: Vec() takes no arguments.
   • C++ (Vec{ .x = 1, .y = 2 }) and C# (new Vec { x = 1, y = 2 }) are correct.

Related: C++ rejects the Point exemplar with "designated initializers cannot be used with a non-aggregate type", because Point declares a constructor.

This is not #297, which covers positional construction in C, Go and Rust and explicitly excludes the member-initialiser form.

Suggested fix / acceptance criteria

When the type is declared in the generation root:

• JavaScript, no constructor: emit Object.assign(new T(), { x: 1, y: 2 }).
• JavaScript, constructor whose parameters match the member names: pass the values positionally, in parameter order.
• Python, no __init__: emit something that sets the attributes, such as a helper or a synthesised keyword __init__.
• Run tests (node, python3) assert the fields hold the given values, both with and without a constructor.