fix: keep __proto__ keys as own properties

[spokodev]

Problem

An own __proto__ property is dropped on a round-trip:

import { stringify } from "javascript-stringify";

const value = JSON.parse('{"__proto__": 42, "a": 1}'); // own enumerable __proto__
stringify(value); // "{__proto__:42,a:1}"

const back = eval("(" + stringify(value) + ")");
Object.getOwnPropertyDescriptor(back, "__proto__"); // undefined  ← lost

{ __proto__: 42 } in an object literal is the prototype setter, not an own property, so the entry disappears (and a __proto__ value that is an object even changes the result's prototype, which then breaks re-stringifying it).

Root cause

quoteKey emits __proto__ as a bare identifier (it is a valid identifier name), producing the prototype-setter form. A quoted string key ("__proto__") would behave the same way — only a computed key creates an own property.

Fix

Emit __proto__ as a computed key:

export function quoteKey(key: PropertyKey, next: Next) {
  if (key === "__proto__") return `[${next(key)}]`;
  return isValidVariableName(key) ? key : next(key);
}

{['__proto__']:42} is an own property and leaves the prototype untouched, matching JSON.parse/structuredClone. Other keys (including constructor, toString) are unaffected.

Test plan

• Added a round-trip test for an own __proto__ key (fails on main, passes with the fix).
• Full suite green (147).