fix(receiver): keep login-item alerts behind fullscreen

[Betafer]

Context

This is a small follow-up to #162. The existing monitor shield uses the same public window level as Notification Center, so a login-item banner created later can still be ordered above the Receiver. On the 2017 21.5-inch iMac, SDL/macOS could also reset the Receiver from the requested shield level back to level 0 after the delayed fullscreen Space transition.

Changes

• use the first window level above kCGStatusWindowLevelKey
• stay below public pop-up-menu and screen-saver/lock levels
• order the Receiver forward only when correcting a changed level
• react to AppKit fullscreen, screen and Retina-backing change notifications
• perform only three bounded post-transition checks (250 ms, 1 s and 2.5 s)
• remove the observers and restore NSNormalWindowLevel on Stop/disconnect

Safety

The patch does not use private APIs or the screen-saver level. Pop-up menus and the lock screen remain above the Receiver. There is no recurring polling: after 2.5 seconds no delayed check remains, and event observers are installed only for an active monitor session.

Validation

• make test and full Receiver build pass on the maint-3.5 base
• full integrated suite passes: 752 assertions/checks plus recovery-helper checks
• native Metal shader/pipeline test passes on the iMac's AMD Radeon Pro 560
• focused AppKit lifecycle harness verifies: no recurring polling, event-driven recovery, Stop restore and stale-retry cancellation
• built and code-signature-verified as x86_64 on a 2017 21.5-inch 4K iMac running macOS 13.7.8
• real sender session accepted after restart with its direct-link TCP connection established
• after all bounded checks had expired, WindowServer confirmed both Receiver surfaces persisted at layer 26; the previous one-shot implementation had fallen back to layer 0