[codex] Fix horizontal scroll swipe transitions

[lynnswap]

Summary

• Add transition hosting/backdrop handling so interactive navigation swipe transitions keep outgoing and incoming views animated correctly, including horizontal NSScrollView roots.
• Update the MiniApp split-navigation previews with normal/scroll content switching and tap-to-push pages for reproducing scroll/navigation gesture conflicts.
• Defer navigation swipe capture for zero-delta and pre-threshold beginnings over scrollable horizontal descendants, with regression coverage for nested scroll views.

Testing

• swift test
• xcodebuild build -project Tools/MiniApp/MiniApp.xcodeproj -scheme MiniApp -destination 'platform=macOS,name=My Mac'
• codex-review --base main clean

Screenshots

• Not captured; MiniApp changes are preview/debug harness updates for gesture reproduction.