[Feature] Add Xcode Preview support for OpenSwiftUI

[Kyle-Ye]

Summary

• Fix Xcode Preview rendering by disabling waitingForPreviewThunks blocking
• SwiftUI's PreviewsInjection.framework calls __previewThunksHaveFinishedLoading() via a two-level namespace binding targeting SwiftUI, so OpenSwiftUI's version is never invoked — causing graph instantiation to block indefinitely
• Implement __previewThunksHaveFinishedLoading() to properly unblock graph hosts
• Add PreviewShims.swift helper to bridge OpenSwiftUI views into #Preview via hosting controllers

[augmentcode]

🤖 Augment PR Summary

Summary: Fixes Xcode Preview rendering for OpenSwiftUI by avoiding a Preview-thunk wait that never unblocks due to SwiftUI’s two-level namespace binding.

Changes:

• Disable `waitingForPreviewThunks` gating so `GraphHost.instantiateIfNeeded()` can instantiate graphs under Xcode Previews.
• Implement `__previewThunksHaveFinishedLoading()` to flush any queued hosts and notify delegates when thunk loading completes.
• Add an example `PreviewShims.swift` bridge that wraps OpenSwiftUI views in hosting controllers for use with `#Preview`.

_{🤖 Was this summary useful? React with 👍 or 👎}

[augmentcode]

Review completed. 1 suggestion posted.

Comment augment review to trigger a new review at any time.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 27.17%. Comparing base (ff8c99a) to head (503c3b8).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/OpenSwiftUICore/Graph/GraphHost.swift	0.00%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #829       +/-   ##
===========================================
+ Coverage   13.83%   27.17%   +13.33%     
===========================================
  Files         630      667       +37     
  Lines       38173    42960     +4787     
===========================================
+ Hits         5283    11675     +6392     
+ Misses      32890    31285     -1605     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.