Form looping issue - Possible to link a page to the "Start page" breaking the entire form

## Context

**Tickets**
https://assistance.cds-snc.ca/helpdesk/tickets/29166
https://cds-snc.freshdesk.com/a/tickets/28853

**Threads:**
https://gcdigital.slack.com/archives/C01GFD150MC/p1784313052405519
https://gcdigital.slack.com/archives/C03T4FAE9FV/p1781873532770719

**Screencapture**
https://github.com/user-attachments/assets/50d6d45a-0c52-40ad-ba27-82595cd3d65e

We should be catching this endless loop error with nothing to get you to the end of the form.

## Description

In the branching view of a multi-page form, select a page and link it to go to the start page. The app breaks and you cannot undo / go back.

## Form file

[test-v1-2026-09-01.json](https://github.com/user-attachments/files/31706712/test-v1-2026-09-01.json)

Start in the branching view for multi-page forms and reproduce the loop using the attached test-v1-2026-09-01.json form file. Trace how linking a page to the start page is handled; done means the invalid endless loop is caught and the form remains usable, with a way to undo or return to editing.

Written by the indexing model from the issue text.