feat(render): add compositing groups

[Kyle-Ye]

Summary

• Add compositingGroup() and its renderer effect, with DocC usage guidance.
• Add shared text and overlapping-circle examples with snapshot tests for group opacity.

Both snapshot cases passed against SwiftUI references on iOS 18.5.

Stack

Depends on #1125.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 30.79%. Comparing base (2816e93) to head (631bf3b).

Files with missing lines	Patch %	Lines
...Render/RendererEffect/CompositingGroupEffect.swift	0.00%	5 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##           feature/effect-styles    #1126   +/-   ##
======================================================
  Coverage                  30.79%   30.79%           
======================================================
  Files                        876      877    +1     
  Lines                      70859    70864    +5     
======================================================
+ Hits                       21820    21825    +5     
  Misses                     49039    49039           

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[tuist]

🛠️ Tuist Run Report 🛠️

Tests 🧪

Scheme	Status	Ran test modules	Commit
OpenSwiftUI	✅	7	9b4f3dd05

Builds 🔨

Scheme	Status	Duration	Commit
OpenSwiftUI	✅	1m 34s	9b4f3dd05

[Kyle-Ye]

/uitest all osui-iag update only-testing=OpenSwiftUIUITests/CompositingGroupEffectUITests