Fix downlevel async super in default parameters

[Copilot]

Downlevel async emit could leave super.m() inside the inner generator function when used in a default parameter. That produced invalid JavaScript because super is not legal in the generated function*.

• Transform fix

  • Tracks super accesses before transforming non-simple async parameter lists.
  • Applies the existing async super substitution to inner generator parameters as well as the body.

• Regression coverage

  • Adds a compiler emit test for both async arrow and async method default parameters:

class C extends B {
    f() {
        const g = async (b = super.m()) => b;
        return g();
    }

    async h(b = super.m()) {
        return b;
    }
}

These now downlevel default initializers to _super.m.call(this) inside the generated function*.