Add ViewGraphGeometryObservers API

[Kyle-Ye]

No description provided.

[augmentcode]

🤖 Augment PR Summary

Summary: Introduces a reusable geometry-observer container for the view graph, intended to track size changes for specific layout proposals.

Changes:

• Added ViewGraphGeometryObservers and ViewGraphGeometryMeasurer to measure sizes by proposal and track pending/stable transitions.
• Implemented a small state machine (Storage) to detect changes and surface “pending” notifications.
• Exposed ViewGraph.sizeThatFitsObservers as a package property and added SizeThatFitsMeasurer (+ SizeThatFitsObservers typealias).
• Relaxed access on layoutComputer and rootViewInsets to support the new measurer.
• Added tests covering observer add/remove exclusivity behavior (with a placeholder callback test).

Technical Notes: The new API is generic over a measurer, enabling reuse for other geometry signals beyond sizeThatFits.

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[augmentcode]

Review completed. 3 suggestions posted.

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[codecov]

Codecov Report

❌ Patch coverage is 47.36842% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 28.76%. Comparing base (b744311) to head (9664d79).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...UICore/View/Graph/ViewGraphGeometryObservers.swift	54.09%	28 Missing ⚠️
Sources/OpenSwiftUICore/View/Graph/ViewGraph.swift	20.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #695      +/-   ##
==========================================
+ Coverage   28.75%   28.76%   +0.01%     
==========================================
  Files         588      589       +1     
  Lines       35701    35775      +74     
==========================================
+ Hits        10266    10291      +25     
- Misses      25435    25484      +49     

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