Hydration cleanup writes i18nNodes/dehydratedIcuData onto user data stored in @let #71929

Which @angular/* package(s) are the source of the bug?

Don't known / other

Is this a regression?

Yes

Description

After hydration, Angular adds i18nNodes and dehydratedIcuData (both undefined) to an object in a @let array. The app does not use i18n.

@let list = todos;
@for (todo of list; track todo.id) {
  <p>{{ todo.title }} ({{ $index + 1 }} of {{ list.length }})</p>
}

todos = Array.from({ length: 8 }, (_, i) => ({ id: i, title: 'todo ' + i }));

constructor() {
  afterNextRender(() => setInterval(() => console.log(this.todos), 5000));
}

On a full load of the server-rendered page, todos[6] is logged as { id: 6, title: 'todo 6', i18nNodes: undefined, dehydratedIcuData: undefined }.

Expected: Angular doesn't modify application data.

Reading list inside the @for body (list.length) is needed to reproduce.

Please provide a link to a minimal reproduction of the bug

https://github.com/swseverance/angular-hydration-let-array-mutation (live: https://ng-hydration-let-mutation--ng-hydration-let-mutation.us-central1.hosted.app)

Please provide the exception or error you saw

ERROR TypeError: Cannot add property i18nNodes, object is not extensible
    at cleanupI18nHydrationData
    at cleanupLView
    at cleanupLView
    at cleanupLView
    at cleanupLContainer
    at cleanupLView
    at cleanupDehydratedViews

Please provide the environment you discovered this bug in (run ng version)

Angular CLI       : 22.2.5
Angular           : 22.2.5
Node.js           : 24.20.0
Package Manager   : npm 11.19.0
Operating System  : darwin x64

Anything else?

The error above is what we saw in our app, where the array was deep-frozen (NgRx dev-mode strictActionImmutability).