codegen: skip composable .ctor on instance interfaces

[yeelam-gordon]

Summary

Unsealed WinRT runtime classes expose a derived-from constructor on the default/required instance interface whose CLR method name is literally .ctor. It follows the COM aggregation pattern and is only meant to be invoked by a host framework (in practice XAML). Codegen used to fall through to the generic instance-method path and emit invalid syntax: def .ctor(self) -> None: in Python and .ctor(): void { ... } in TypeScript. Full-WinAppSDK smoke previously reported 62 affected files, all in Microsoft.UI.Xaml.*.

What changed

Early-return at the entry of the three per-method emitters:

• project_instance_method (TS) -- project.rs
• generate_iface_instance_method (Python) -- py_method.rs
• emit_method_stub (Python .pyi stub) -- python_stub.rs

The .add_method(.ctor, ...) entry is still recorded in interface registration so vtable indices and parameterized-IID computation remain correct. Factory .ctor(args) (legitimate class construction) still routes through project_factory_method / generate_factory_method_invoke unchanged, and delegate .ctor + Invoke is still short-circuited to project_delegate / generate_delegate.

Verification

• New tests/composable_ctor_test.rs (3 tests) pins all three behaviors: no .ctor declaration in TS / Python / .pyi output, and the interface registration still reserves a vtable slot per method.
• Existing snapshot/cleanliness/tsc tests still pass -- Uri output unchanged.
• Full Microsoft.UI.Xaml.Controls.Button generation: 383 .js / 383 .d.ts / 383 .py / 383 .pyi files, all syntactically valid:
  • python -m py_compile on every .py: 0 errors / 383 files
  • ast.parse on every .pyi: 0 errors / 383 files
  • 36 .ctor registrations preserved across the namespace (vtable correctness)

Out of scope

Full fix for actually invoking the composable constructor still requires implementing COM aggregation subclassing and XAML hosting -- out of scope until we take that on. This PR only stops codegen from emitting syntactically invalid output.