core-js 3.40.0: Demonstrate that core-js polyfilled Object.groupBy returns a null-prototype object rather than a standard Object instance.

# core-js Object.groupBy Prototype Trait Notes

## Search Findings
Searching for existing solution coverage returned three published samples focusing on `@babel/preset-env` integration with `core-js` (specifically `useBuiltIns` default behavior, engine bugfix transforms, and target query edge cases).

## Prototype Behavior
In `core-js` (matching the ES2024 specification), `Object.groupBy` returns a prototype-less object created with a `null` prototype.

Code written under the naive belief that `Object.groupBy` returns a standard JavaScript object inheriting from `Object.prototype` fails loudly at runtime with `TypeError: grouped.toString is not a function` when calling inherited methods like `.toString()` or `.hasOwnProperty()`, and `grouped instanceof Object` evaluates to `false`.

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      "name": "npm-core-js-d1",
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      }
    },
    "node_modules/core-js": {
      "version": "3.40.0",
      "resolved": "https://registry.npmjs.org/core-js/-/core-js-3.40.0.tgz",
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{
  "name": "npm-core-js-d1",
  "description": "CSX sample demonstrating Object.groupBy return prototype behavior in core-js",
  "version": "1.0.0",
  "type": "commonjs",
  "dependencies": {
    "core-js": "^3.40.0"
  }
}

import 'core-js/actual/object/group-by.js';
import assert from 'node:assert/strict';

// Feature under test: core-js polyfilled Object.groupBy
const items = [
  { name: 'apple', category: 'fruit' },
  { name: 'carrot', category: 'vegetable' },
  { name: 'banana', category: 'fruit' }
];

const grouped = Object.groupBy(items, item => item.category);

// 1. Prototype of Object.groupBy result is null, not Object.prototype
assert.strictEqual(
  Object.getPrototypeOf(grouped),
  null,
  'Object.groupBy result prototype must be null'
);

// 2. instanceof Object evaluates to false
assert.strictEqual(
  grouped instanceof Object,
  false,
  'Object.groupBy result must not be an instance of Object'
);

// 3. Standard Object.prototype methods like toString are undefined
assert.strictEqual(
  grouped.toString,
  undefined,
  'Object.groupBy result must not inherit toString'
);

// 4. Invoking toString on the result throws TypeError
assert.throws(
  () => grouped.toString(),
  TypeError,
  'Invoking toString on Object.groupBy result must throw TypeError'
);

// 5. Grouping logic operates as expected
assert.deepStrictEqual(grouped.fruit, [
  { name: 'apple', category: 'fruit' },
  { name: 'banana', category: 'fruit' }
]);
assert.deepStrictEqual(grouped.vegetable, [
  { name: 'carrot', category: 'vegetable' }
]);