[codex] Restore iOS app and add language chat

[rudrankriyam]

Summary

• Restore the iOS CoreAILab app target while moving the macOS app flow back to CoreAILabMac.
• Add a transcript-based Core AI language chat experience for imported CoreAILanguageModel bundles.
• Add pinned language presets for Qwen3 4B and Gemma 3 4B/12B, while keeping iOS recipe claims limited to the Qwen recipes present in the current Apple catalog.
• Update README, AGENTS guidance, macOS CI, and the diarization integration script for the restored scheme split.

Why

The iOS app gives Core AI Lab the “this runs on iPhone” surface, and the language workspace now feels like the sibling Foundation Models Lab chat UI while staying honest about the Core AI runtime boundary.

Validation

• xcodebuild -project CoreAIFrameworkLab.xcodeproj -list
• xcodebuild -project CoreAIFrameworkLab.xcodeproj -scheme CoreAILabMac -destination 'platform=macOS,arch=arm64' -derivedDataPath ./build/Xcode27 build
• xcodebuild -project CoreAIFrameworkLab.xcodeproj -scheme CoreAILab -destination 'generic/platform=iOS' -derivedDataPath ./build/Xcode27 CODE_SIGNING_ALLOWED=NO build
• xcodebuild -project CoreAIFrameworkLab.xcodeproj -scheme CoreAILabMac -destination 'platform=macOS,arch=arm64' -derivedDataPath ./build/Xcode27 test -only-testing:CoreAILabMacTests/AppleLanguageWorkspaceModelTests
• git diff --check