bug: Stacking order does not respect layout order when zIndex is absent

[veracioux]

Title. Originally discovered this in opencode when cycling between parent session and subagent session:

https://github.com/user-attachments/assets/39f729aa-3770-446f-8d76-0c2c04192461

Will have a PR ready shortly.

Minimalistic reproduction repo:

https://github.com/veracioux/opentui-332-repro

• Look at index.tsx, then run bun dev and you'll see the issue.