fix: serialize an Invalid Date instead of throwing

[spokodev]

Problem

serialize (and the /json stringify) throws on an Invalid Date, even though native structuredClone — the API this package ponyfills — clones it without issue:

import {serialize, deserialize} from '@ungap/structured-clone';

structuredClone(new Date(NaN));        // Invalid Date, preserved
deserialize(serialize(new Date(NaN))); // RangeError: Invalid time value

An Invalid Date is a legitimate value (e.g. the result of new Date('not a date')), so any object graph that happens to contain one crashes the clone. The README documents no Invalid-Date caveat (the only stated limitation is unsupported array elements becoming null).

Cause

Date.prototype.toISOString() throws a RangeError when the date's time value is NaN, and the DATE case calls it unconditionally:

case DATE:
  return as([TYPE, value.toISOString()], value);

The deserialize side already handles it: it does new Date(value), which faithfully revives an Invalid Date.

Fix

Store an empty string for an Invalid Date:

case DATE:
  return as([TYPE, isNaN(value.getTime()) ? EMPTY : value.toISOString()], value);

EMPTY ('') is chosen deliberately over NaN/getTime(): it is JSON-safe, so the /json path round-trips too (NaN would serialize to null, and new Date(null) revives to epoch 0), and new Date('') revives to an Invalid Date — matching the deserialize side. Valid dates are unaffected.

Tests

Added round-trip assertions for an Invalid Date on both the record API and the /json API. They throw on main and pass with the fix; the rest of the suite is unchanged.

(The minified structured-json.js browser bundle is a build artifact regenerated by npm run build, so it is left out of this diff.)