[codex] Recover status item after display changes

[Llldmiao]

Fixes #997.

Summary

• Detect status items whose backing window is attached to a detached/non-current screen.
• Observe NSApplication.didChangeScreenParametersNotification and debounce display-change recovery.
• Recreate status items when a display is removed while a visible item exists, or when a visible item snapshot is blocked.
• Add focused coverage for detached-screen snapshots and display-removal recovery decisions.

Root Cause

The startup recovery added in a75d276f catches status items that fail to materialize shortly after launch, but CodexBar did not observe runtime display topology changes. If the external display hosting the NSStatusItem is unplugged, AppKit can leave the app process alive while the status item is still associated with a removed screen/menu bar.

Validation

• git diff --check passes.
• swift test --filter MenuBarVisibilityWatcherTests passes with Xcode 26.5 / Apple Swift 6.3.2.

Notes

This PR intentionally targets the runtime hot-unplug case. It is complementary to the startup recovery in #988, the Tahoe allow-list guidance in #945, and the earlier macOS 26.4 visibility fix in #849.

[steipete]

Thanks for the focused fix. I landed this on main with a small follow-up for the debounce cancellation path so stale display-change checks cannot run after a newer notification cancels them.

Landed as fcbd46f.

Verified with:

• swift test --filter MenuBarVisibilityWatcherTests
• swift test --filter StatusItemControllerSplitLifecycleTests
• make check
• codex-review --mode auto