Title: Temporal.Now is non-configurable (frozen namespace) — deviates from spec, breaks reconfiguration tools

Summary
The global Temporal namespace produced by this polyfill is frozen, so Temporal.Now is non-reconfigurable. This deviates from the Temporal spec and breaks tooling that legitimately redefines Temporal.Now (e.g. @sinonjs/fake-timers ≥15.4, and therefore Vitest/Jest fake timers).

Observed descriptor

Object.getOwnPropertyDescriptor(Temporal, 'Now')
// { value: …, writable: false, enumerable: true, configurable: false }
Object.isExtensible(Temporal) // false

(Measured on @js-temporal/[email protected].)

Expected per spec
Temporal proposal §1.3.1 assigns Temporal.Now no custom attributes, so the standard built-in default applies: { writable: true, enumerable: false, configurable: true }. Two mismatches:

• configurable should be true (this is the breaking one)
• enumerable should be false (currently true)

For comparison, native V8 (Node ≥26) and temporal-polyfill both expose Temporal.Now as configurable: true.

Impact / repro

import '@js-temporal/polyfill'              // installs frozen Temporal globally
import { install } from '@sinonjs/fake-timers'
install()                                    // throws:
// TypeError: Cannot redefine property: Now

fake-timers builds a clock-backed Temporal by copying the namespace's own props and then Object.defineProperty(…, 'Now', …). Because the copied Now is non-configurable, the redefine throws. Any consumer that relies on the spec-guaranteed configurability hits this.

Suggested fix
Define Temporal.Now with { writable: true, enumerable: false, configurable: true } and avoid Object.freeze/non-extensible on the Temporal namespace (or at least leave Now reconfigurable), matching the spec and native engines.

Notes

• Only affects the polyfill installed on the global object; local import { Temporal } from '@js-temporal/polyfill' is unaffected.
• Environment: @js-temporal/[email protected], Node v24.