Add initial Xcode 26 SDK support

[Kyle-Ye]

Avoid exposing UIKit/AppKit dependency on OpenSwiftUI_SPI which will be imported by OpenSwiftUICore.

UIKit/AppKit will exported SUICore since SDK 26. And many Animatable extension will have conflict due to such usage.

The new design is as follows:

• OSUI -> COSUI
• OSUICore -> OSUI_SPI

Note: This only add initial Xcode 26 support (No CI pipeline yet). And it does not imply iOS 26 and macOS 26 support. You should still use iOS 18.5 and macOS 15 as the running destination.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 30.26%. Comparing base (f84f25e) to head (80cd399).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #634      +/-   ##
==========================================
+ Coverage   30.24%   30.26%   +0.01%     
==========================================
  Files         552      552              
  Lines       32970    32970              
==========================================
+ Hits         9973     9977       +4     
+ Misses      22997    22993       -4     

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