[codex] Support iOS 16 UIKit root contract

[fannnzhang]

What changed

• Lowered the iOS app, widget extension, tests, Xcode project, and UniFFI build defaults from iOS 17.0 to iOS 16.0.
• Replaced iOS 17-only SwiftUI APIs in production app/widget sources with iOS 16-compatible project wrappers or UIKit-backed implementations.
• Added guardrails to verify-roadmap-architecture-constraints.sh for iOS 16 deployment and forbidden iOS 17 SwiftUI APIs.
• Documented the UIKit/AppKit migration contract and the next root/list/auth/profile workstreams.

Why

The next iOS architecture phase targets iOS 16 and production UIKit/Texture ownership, with SwiftUI limited to WidgetKit, Developer Tools, and tracked transitional bridges. This PR establishes the compatibility floor and guardrails before larger root-shell and screen migrations begin.

Validation

• scripts/verify-roadmap-architecture-constraints.sh
• xcodegen generate
• xcodebuild -project native/ios-app/Fire.xcodeproj -scheme Fire -destination 'platform=iOS Simulator,id=5318776E-FF02-4468-8549-DFF919ED16E0' -derivedDataPath /tmp/fire-ios16-uikit-contract-full2 CODE_SIGNING_ALLOWED=NO build

Notes

• Existing local references/fluxdo and untracked references/dexo/ changes are not part of this PR.
• Remaining build warnings are pre-existing Texture/Swift diagnostics unrelated to the iOS 16 contract.