Fix 0.19.0 iOS binary package usage

[Kyle-Ye]

Summary

Fixes a 0.19.0 iOS binary package usage issue where downstream apps could fail to import OpenSwiftUI from the prebuilt XCFramework.

The UIKit geometry hook remains implemented, but is now hidden from the public Swift interface with SPI.

Root Cause

The iOS simulator XCFramework public .swiftinterface exposed _UIHostingView._geometryChanged(_:forAncestor:) as an open UIKit override. Downstream consumers validated that declaration against UIKit's public module interface, where this selector is not declared, so importing OpenSwiftUI could fail before the app target built.

Keeping the override open is still required for the implementation to compile, so this change marks it as SPI instead of lowering the access level.

[augmentcode]

🤖 Augment PR Summary

Summary: Hides _UIHostingView._geometryChanged(_:forAncestor:) from the public Swift interface by marking the override as SPI.

Why: Avoids downstream import failures when validating the prebuilt iOS simulator XCFramework’s .swiftinterface against UIKit, while keeping the required open override for compilation.

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[augmentcode]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 25.93%. Comparing base (859ce24) to head (5111170).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #931   +/-   ##
=======================================
  Coverage   25.92%   25.93%           
=======================================
  Files         720      720           
  Lines       50774    50774           
=======================================
+ Hits        13163    13166    +3     
+ Misses      37611    37608    -3     

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