[Feature] Add missing DisplayList API

[Kyle-Ye]

Summary

• Implement missing DisplayList APIs audited against SwiftUICore 6.5.4 Hopper assembly.
• Add aggregate cache handling for properties across init/append/transform paths.
• Implement traversal helpers for identities and RB display lists inline in DisplayList, matching the latest forEachIdentity((Identity, inout Bool) -> Void) -> Bool ABI.
• Add reserveCapacity, translate, version, nextUpdate, and explicit equality behavior.

[augmentcode]

🤖 Augment PR Summary

Summary: This PR fills in missing pieces of the core DisplayList API to better match SwiftUICore 6.5.4 behavior.

Changes:

• Updates audit marker for DisplayList.swift to 6.5.4.
• Implements list construction and mutation paths: init(Item), append, append(contentsOf:), and reserveCapacity, while maintaining aggregate features/properties caches.
• Adds transform to mutate items and recompute cached aggregates, and translate(by:version:) to offset item geometry and advance item versions.
• Adds a computed version across all items and keeps nextUpdate(after:) behavior intact.
• Introduces traversal helpers forEachIdentity and forEachRBDisplayList for identity/RB list discovery through nested display lists.
• Defines explicit DisplayList == to compare items plus cached aggregate state.
• Implements DisplayList.Item.properties aggregation across flattened content, effects (including masks and interpolator layers), and states.
• Wires interpolator-related property aggregation through new properties overrides/forwarders on interpolator-layer structures.
• Removes a placeholder properties implementation from DisplayListPrinter now that it’s implemented on DisplayList.Item.
• Adds unit tests validating property propagation for property effects, masks, flattened content, and states.

Technical Notes: The change centers on keeping features and properties caches consistent across init/append/transform paths and enabling traversal of nested structures for identity/RB-list enumeration.

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[augmentcode]

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[codecov]

Codecov Report

❌ Patch coverage is 27.43363% with 82 lines in your changes missing coverage. Please review.
✅ Project coverage is 25.73%. Comparing base (32b3832) to head (aa5dac5).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...enSwiftUICore/Render/DisplayList/DisplayList.swift	27.43%	82 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #864       +/-   ##
===========================================
+ Coverage   13.74%   25.73%   +11.98%     
===========================================
  Files         637      677       +40     
  Lines       41167    47008     +5841     
===========================================
+ Hits         5660    12099     +6439     
+ Misses      35507    34909      -598     

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