Describe the bug

When you click on the "insert row" button in the example app, new rows get added to the 1940s root, except for those in the 2000s.

Steps to reproduce

Run the tree example app and click on "insert row" a few times.

Expected behavior

Items should be correctly distributed by decade.

Screenshots

Environment

• Operating System: MacOS (I don't think this matters)
• Python version: 3.13.9 (this might matter)
• Software versions:
  • Toga: current main branch

Logs

No response

Additional context

This looks like it was introduced in #4000. I think

if (year := item["year"]) >= 2000: