Enable Preview examples on macOS

[Kyle-Ye]

Summary

• enable the shared Preview examples for native macOS targets
• use an AppKit-compatible size for the Core Animation hosting preview
• document macOS Preview support enabled by Fix Xcode Previews for binary framework distributions #952

Motivation

The shared Preview declarations remained limited to UIKit even after the binary framework linkage fixes made macOS Preview available. This enables the same hosting previews for AppKit while preserving the existing UIKit behavior.

Validation

• verified the final diff is clean

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 14.87%. Comparing base (b92eeb2) to head (ff48805).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #976      +/-   ##
==========================================
- Coverage   14.89%   14.87%   -0.02%     
==========================================
  Files         688      688              
  Lines       46836    46885      +49     
==========================================
- Hits         6975     6974       -1     
- Misses      39861    39911      +50     

☔ View full report in Codecov by Harness.
📢 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.