Add initial async render support

[Kyle-Ye]

No description provided.

[augmentcode]

🤖 Augment PR Summary

Summary: This PR introduces initial async rendering support by allowing parts of the view graph update + display list rendering pipeline to run off the main thread when it is safe to do so.

Changes:

• Refactors the async render thread startup loop in DisplayLink to wait for the async runloop to become available.
• Adds a SwiftUI symbol-locator stub for DisplayList.ViewRenderer.renderAsync and routes async rendering through it when available.
• Implements allowsAsyncUpdate() checks for Attribute/WeakAttribute to gate async updates.
• Implements ViewGraph.updateOutputsAsync and refactors output updating to support async mode with feature gating.
• Adds per-feature state (needsUpdate, skipsAsyncUpdate) stored in the heterogeneous buffer element flags.
• Makes UnsafeHeterogeneousBuffer storage fields private and adjusts iteration/append behavior; tests are updated to avoid relying on private fields by default.

Technical Notes: Async update is conservatively disabled when attributes/preferences/layout observers/features indicate main-thread requirements; when async is permitted, main-thread callbacks are still dispatched via Update.syncMain.

_{🤖 Was this summary useful? React with 👍 or 👎}

[augmentcode]

Review completed. 1 suggestions posted.

Comment augment review to trigger a new review at any time.

[codecov]

Codecov Report

❌ Patch coverage is 7.76699% with 95 lines in your changes missing coverage. Please review.
✅ Project coverage is 14.29%. Comparing base (4e2a39c) to head (3db2e6e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/OpenSwiftUICore/View/Graph/ViewGraph.swift	0.00%	54 Missing ⚠️
.../OpenSwiftUICore/View/Graph/ViewGraphFeature.swift	0.00%	20 Missing ⚠️
.../OpenSwiftUICore/View/Graph/ViewRendererHost.swift	0.00%	9 Missing ⚠️
Sources/OpenSwiftUICore/Graph/GraphHost.swift	0.00%	6 Missing ⚠️
...OpenSwiftUICore/Util/AttributeGraphAdditions.swift	0.00%	3 Missing ⚠️
...UICore/View/Graph/ViewGraphGeometryObservers.swift	0.00%	2 Missing ⚠️
...e/Render/DisplayList/DisplayListViewRenderer.swift	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (4e2a39c) and HEAD (3db2e6e). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (4e2a39c)	HEAD (3db2e6e)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #700       +/-   ##
===========================================
- Coverage   28.86%   14.29%   -14.57%     
===========================================
  Files         589      563       -26     
  Lines       35804    33190     -2614     
===========================================
- Hits        10335     4746     -5589     
- Misses      25469    28444     +2975     

☔ 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.