fix(hosting): hide private platform overrides from module interfaces

[Kyle-Ye]

Summary

• Hide private UIKit and AppKit hosting overrides from public, private, and package Swift interfaces with @_implementationOnly.
• Preserve the required override declarations and Objective-C dispatch for geometry, hit testing, gesture containers, and rendered subview insertion.

Validation

• Verified interface emission and unchanged Swift methods and Objective-C thunks for the UIKit and AppKit insertion hooks.
• Verified rendered subview insertion through Objective-C selectors with focused runtime probes on iOS 18.5, iOS 26.2, and macOS.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 33.10%. Comparing base (5af2b2b) to head (40b3789).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1089      +/-   ##
==========================================
+ Coverage   28.87%   33.10%   +4.23%     
==========================================
  Files         750      803      +53     
  Lines       55271    63659    +8388     
==========================================
+ Hits        15958    21076    +5118     
- Misses      39313    42583    +3270     

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[tuist]

🛠️ Tuist Run Report 🛠️

Tests 🧪

Scheme	Status	Cache hit rate	Tests	Skipped	Ran	Commit
OpenSwiftUI	❌	0 %	1241	0	1241	30d6c4ace

Failed Tests ❌

• OpenSwiftUI: 1 failed test (View all)

• maxWidthInfinityExpandsToParentProposal() · OpenSwiftUICoreTests · FlexFrameLayoutTests
  (size → (200.0, 262.0)) == (CGSize(width: 200, height: 200) → (200.0, 200.0))

Builds 🔨

Scheme	Status	Duration	Commit
OpenSwiftUI	✅	1m 17s	30d6c4ace