Add underscored API documentation comments

[Kyle-Ye]

Agent Summary

Adds missing documentation comments for selected underscored public API surfaces.

The update places comments on the primary type or protocol declarations where appropriate, and avoids repeating identical comments on default implementations. It also keeps the wording aligned with OpenSwiftUI naming.

Validation:

• Ran git diff --check.
• Checked newly added comments for stale SwiftUI wording and doc:// links.
• No build was run; this is a Swift documentation-comment-only change.

[augmentcode]

🤖 Augment PR Summary

Summary: This PR improves API documentation coverage for several underscored but public OpenSwiftUI/OpenSwiftUICore types and members.

Changes:

• Added doc comments to underscored protocols/types like _PerformanceTest, _AnimationModifier, _ColorMatrix, and various layout/effect helpers.
• Documented underscored member requirements such as Commands._resolve(into:), TransactionKey._valuesEqual(_:_:), and LayoutSubview._trait(key:).
• Added parameter docs for benchmarking APIs (e.g., scroll testing and benchmark lifecycle hooks).

Technical Notes: Changes are documentation-only and aim to align wording with OpenSwiftUI naming and swift-docc conventions.

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[augmentcode]

Review completed. 2 suggestions posted.

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

[augmentcode]

PPT is an unexplained acronym here; consider expanding it or linking to the test harness so the rendered docs are understandable outside this repo.

Severity: low

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[augmentcode]

Minor grammar issue: “The result of applying to a color…” reads like it’s missing the object (e.g., applying it to a color), which can make the doc sentence confusing.

Severity: low

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 26.33%. Comparing base (ec8d089) to head (414b1f6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #909      +/-   ##
==========================================
- Coverage   26.35%   26.33%   -0.02%     
==========================================
  Files         710      710              
  Lines       49537    49537              
==========================================
- Hits        13053    13048       -5     
- Misses      36484    36489       +5     

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