Wrap #Preview body in @ViewBuilder func to accept if #available et al

[obj-p]

Summary

• Fixes a bug where BridgeGenerator emitted invalid Swift when a #Preview body started with if #available / if #unavailable, used a leading let/var, or had if/switch branches with different concrete View types. Reported against Prism iOS (CatalogChatThread.swift:360).
• The root cause: we were pasting the closure body as a bare expression argument to AnyView(...), stripping the @ViewBuilder context that Xcode's #Preview macro provides via DeveloperToolsSupport.Preview.init.
• The fix: declare a nested @ViewBuilder func __previewBody() -> some SwiftUI.View inside the generated bridge and call it from AnyView(__previewBody()). Same runtime semantics as Xcode's #Preview, zero added view types in the render tree.

Why not Group { }?

Group would have worked functionally, but it has a canvas-level quirk in the legacy PreviewProvider API where Xcode's preview inspector walks into a top-level Group and renders each child as a separate preview card. That quirk doesn't apply to our runtime render path (we go through NSHostingView / UIHostingController, not Xcode's canvas) or to the new #Preview macro, but seeing Group in generated bridge code is a code smell that invites confusion. A nested @ViewBuilder function gives unambiguous intent and the same compiler machinery Xcode's macro uses.

The rationale is documented inline in BridgeGenerator.swift.

Cases this now handles

Pattern	Previously	Now
MyView()	✅ worked	✅ works
if #available(iOS X, *) { A() } else { B() }	❌ invalid Swift	✅ tested end-to-end
let model = Model(); MyView(model: model)	❌ silent bug	✅ tested end-to-end
switch state { case .a: A(); case .b: B() } with different branch types	❌ type error	✅ (via _ConditionalContent)
if cond { A() } else { B() } with different branch types	❌ type error	✅ (via _ConditionalContent)

Test plan

• swift test --filter "BridgeGeneratorTraits" — 40/40 pass, including:
  • fullPipelineWithIfAvailable — real Compiler proves if #available body compiles
  • fullPipelineWithMultiStatement — real Compiler proves let x = ...; MyView() body compiles
  • Source-level assertions that @ViewBuilder func __previewBody() precedes the body it wraps
  • Ordering assertion that trait modifiers land on __previewBody(), not inside its body
• swift test --filter "PreviewsCoreTests" — 141/141 pass (no regressions in other test suites)
• swift build — clean
• swift-format lint --strict — clean
• Smoke test against the original repro (Prism iOS CatalogChatThread.swift:360) — reporter to verify

🤖 Generated with Claude Code