Enhance ZIndex implementation with availability annotations and improved tests

[Kyle-Ye]

Summary

This PR enhances the ZIndex implementation by adding proper availability annotations and comprehensive test coverage.

Testing

• ✅ Unit tests pass with proper trait collection validation
• ✅ UI snapshot tests verify correct visual layering

Checklist

• Code follows SwiftUI API compatibility patterns
• Tests use swift-testing framework with proper assertions
• Availability annotations match OpenSwiftUI versioning scheme

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 27.50%. Comparing base (a29a9cc) to head (5ce4276).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #576   +/-   ##
=======================================
  Coverage   27.49%   27.50%           
=======================================
  Files         526      526           
  Lines       30622    30622           
=======================================
+ Hits         8421     8423    +2     
+ Misses      22201    22199    -2     

☔ 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.

[Kyle-Ye]

The DL is not aligned currently:

SwiftUI
(display-list
  (item #:identity 4 #:version 11
    (frame (50.0 104.0; 100.0 100.0))
    (effect
      (transform rotation(SwiftUI._RotationEffect.Data(angle: SwiftUI.Angle(radians: 0.7853981633974483), anchor: (50.0, 50.0))))
      (item #:identity 3 #:version 10
        (frame (0.0 0.0; 100.0 100.0))
        (content-seed 21)
        (color #FF3B30FF))))
  (item #:identity 2 #:version 12
    (frame (50.0 -4.0; 100.0 100.0))
    (content-seed 25)
    (color #FFCC00FF)))

OpenSwiftUI
(display-list
  (item #:identity 4 #:version 2
    (frame (0.0 104.0; 200.0 96.0))
    (effect
      (transform rotation(SwiftUI._RotationEffect.Data(angle: SwiftUI.Angle(radians: 0.7853981633974483), anchor: (100.0, 48.0))))
      (item #:identity 2 #:version 1
        (frame (50.0 -2.0; 100.0 100.0))
        (content-seed 3)
        (color #FF3B30FF))))
  (item #:identity 1 #:version 3
    (frame (50.0 -2.0; 100.0 100.0))
    (content-seed 7)
    (color #FFCC00FF)))