Support UIKit bodies in #Preview blocks

[obj-p]

Summary

• #Preview { ExampleUIView() } blocks failed to compile because the generated bridge wrapped every body in a @ViewBuilder func __previewBody() -> some SwiftUI.View, which rejects UIView/UIViewController returns.
• New PreviewBridgeSource template emits __PreviewBridge.wrap(_:) with a @ViewBuilder generic overload for SwiftUI.View plus non-generic overloads for UIView/UIViewController that auto-wrap in UIViewRepresentable/UIViewControllerRepresentable. Swift's overload resolution picks the right path from the body's return type — same approach Xcode's first-party #Preview macro uses.
• Helper is platform-gated at generation time via PreviewPlatform rather than #if canImport(UIKit) because UIKit is visible on macOS SDKs for Catalyst interop. AppKit bodies on macOS remain out of scope.

Why

Reported failure when rendering #Preview { ExampleUIView(deps: deps) } inside withDependencies: error: static method 'buildExpression' requires that 'ExampleUIView' conform to 'View'. The @ViewBuilder wrapper Swift-constrained every preview body to SwiftUI views even though many users write UIKit previews.

Known limitations (same as Xcode's #Preview for UIKit)

• Multi-statement UIKit bodies require an explicit return — UIKit has no @ViewBuilder equivalent.
• A UIView subclass that also conforms to SwiftUI.View would be ambiguous; left undocumented to avoid @_disfavoredOverload (an underscore-prefixed compiler-internal attribute).
• Protocol-existential returns (any UIKitProtocol) won't be auto-wrapped; users can add an explicit representable.

Test plan

• swift test --filter PreviewsCoreTests — 207 tests green (204 pre-existing + 3 new).
• New full-pipeline compile tests exercise the actual Swift compiler against platform: .iOS bodies:
  • fullPipelineUIViewBody — #Preview { ExampleUIView() }
  • fullPipelineUIViewControllerBody — #Preview { ExampleVC() }
  • fullPipelineSwiftUIBodyIOS — regression guard that SwiftUI bodies still compile via the new __PreviewBridge.wrap path.
• fullPipelineWithIfAvailable and fullPipelineWithMultiStatement still pass unchanged (preserve @ViewBuilder semantics: if #available, leading let, branches with different concrete types).
• examples/spm/Sources/ToDo/UIKitPreview.swift added as an end-to-end fixture; builds cleanly on macOS (guarded with #if canImport(UIKit)).
• Manual: run /integration-test against the iOS simulator to render the new UIKit example end-to-end.

🤖 Generated with Claude Code