… to static private field (#21950)

fixes #21874

`transform_static_assignment_expression` took `assign_expr.right` once at the top of the function (correct for the `=` branch), then took it a second time inside the `else` branch that handles compound operators. The second `take_in` returned a `NullLiteral` dummy, so `??=` / `&&=` / `||=` (and the binary ops) on a static private field where the receiver isn't a direct class reference — e.g. `this.#shared ??= 1` inside a static method — emitted `null` in place of the RHS:

```js
// before
class Singleton {
  static #shared;
  static shared() {
    this.#shared ??= 1;
    return this.#shared;
  }
}

// transformer output (broken):
_assertClassBrand(Singleton, this, _shared)._ ?? (_shared._ = _assertClassBrand(Singleton, this, null));
//                                                                                              ^^^^ should be 1
```

Removes the redundant inner `let class_ident = …; let value = assign_expr.right.take_in(…);` shadowing so the compound-operator branches use the already-captured outer `value` (the real RHS) when building the `_assertClassBrand(Class, object, value)` call. Output now matches Babel.

Adds a `private-static-logical-assignment` conformance fixture covering `??=`, `&&=`, and `||=`, plus an `exec.js` to verify runtime behavior.