feat(ios): add --catalyst to build Mac Catalyst apps

[farfromrefug]

Summary

ns build ios --catalyst rebuilds the existing iOS target against the macOS SDK with the Mac Catalyst variant, producing a native macOS .app in platforms/ios/build/<Configuration>-maccatalyst.

Mac Catalyst is a variant of the iOS platform rather than a platform of its own, so this reuses the iOS target instead of introducing a new one. Four things it has to get right:

• destination, and no -sdk. platform=macOS,variant=Mac Catalyst selects the variant; passing an SDK on top of it makes xcodebuild fall back to iphoneos, so the Catalyst path supplies BUILD_DIR/SHARED_PRECOMPS_DIR directly instead of going through getBuildCommonArgs. Without that the .app lands in DerivedData and the CLI reports "No .app found".
• SUPPORTS_MACCATALYST=YES is forced, since the runtime template only sets the legacy SUPPORTS_UIKITFORMAC alias.
• signing goes through -allowProvisioningUpdates. Catalyst needs a macOS provisioning profile, which $iOSSigningService cannot select; Xcode creates it automatically.
• the deployment target is clamped, not validated. Catalyst starts at iOS 13.1. Failing a project that still targets iOS 12 would force it to drop iOS 12 users just to get a Mac build, so the target is raised for this build only, with a warning. MACCATALYST_DEPLOYMENT_TARGET is passed alongside because the runtime's metadata generator reads it and older runtimes crash when it is unset.

Not included: running the result. ns run <platform> goes through the LiveSync pipeline, which is built around Mobile.IDevice — a Catalyst app is a local .app you open, not a device. Wiring that up means a Mac pseudo-device implementing IDeviceApplicationManager (24 members) and IDeviceFileSystem (34 members) plus device discovery, which belongs in its own PR. For now: open platforms/ios/build/Debug-maccatalyst/<name>.app.

Testing

Built a real app (OSS Weather, iOS deployment target 12.0) from a clean platforms/ios:

$ ns build ios --catalyst
Mac Catalyst requires iOS 13.1 or higher. Building the Mac Catalyst app with IPHONEOS_DEPLOYMENT_TARGET=13.1 instead of the project's 12.0.
Project successfully built.
The build result is located at: .../platforms/ios/build/Debug-maccatalyst/ossweather.app

The .app launches on Apple Silicon as a native macOS app — menu bar, resizable window, weather data loading. All 13 CocoaPods dependencies compiled for arm64-apple-ios-macabi without changes. tsc --noEmit and npm run build are clean.

Note this also needs the runtime-side fixes in Akylas/nativescript-ios-runtime#4 — without them the metadata generator crashes or emits metadata with no UIKit.