Align full-stack regression coverage

[brsbl]

Summary

• update the sidebar-tab full-screen regression to assert the shipped Full Screen / Exit Full Screen interaction and pressed state
• keep the same-slot restoration guarantee covered after the unified control rename

Verification

• focused @bb/app pane/sidebar/browser suite: 9 files, 67 tests passed
• full-stack branch app: thread and browser-tab expansion/restoration, browser chrome, nested split resize, side-panel availability, and new-thread terminal surface verified

Stack parent: #967. Do not merge independently.

[brsbl]

This empty, closed layer was removed from the GitHub stack after its regression-only change was absorbed into #967 during rebasing. The active top layer is now #970, position 9 of official stack #971. No code was dropped.