Add Apple Vision Pro (visionOS) support

[powerje]

Summary

• Add visionOS as a build destination with native flat-window support (not "Designed for iPad" compatibility mode)
• Add .visionOS(.v26) platform to all 20 vendored SPM packages
• Guard unavailable APIs: UIScreen, inputAccessoryView, status bar, vibration, device orientation, UIAlertView, UIActionSheet, and deprecated UIKit constants
• Stub DAKeyboardControl entirely on visionOS (keyboard tracking APIs unavailable)
• Set preferred visionOS window size to 1280x960
• Hide sidebar by default on visionOS (toggle with Cmd+L); tile layout when shown
• Enable Mac Catalyst menu bar (Cmd+N, Cmd+R, etc.) on visionOS