Add accent color API support

[Kyle-Ye]

Agent Summary

Add accent color API coverage and macOS system accent plumbing.

What Changed

• Added accent color environment support, including default accent provider hooks, system accent value storage, and catalog-backed resolution.
• Added Appearance and CoreUISystemCatalogColorName support used by appearance asset lookups and appearance catalog shape styles.
• Wired the OpenSwiftUI platform environment to use the default accent provider and macOS system accent value provider.
• Added the macOS accent provider mapping from system accent values to catalog color names, including multicolor and hardware fallbacks.

Validation

• swift build --target OpenSwiftUICore
• swift build --target OpenSwiftUI

[augmentcode]

🤖 Augment PR Summary

Summary: This PR adds initial accent color API support to OpenSwiftUI, including environment plumbing and macOS system accent integration.

Changes:

• Introduces Color.accentColor plus an accentColor environment value, and adds a deprecated View.accentColor(_:) modifier for API compatibility.
• Adds default accent provider hooks (DefaultAccentColorProvider) and system accent value/provider infrastructure (SystemAccentValue).
• Wires platform environment configuration to install the default accent provider, and installs a macOS system accent provider.
• Adds macOS Appearance/_ResolvedAppearance scaffolding to support catalog-backed appearance lookups.
• Introduces CoreUISystemCatalogColorName and an appearance-backed shape style for resolving CoreUI system catalog colors.
• Updates tint fallback behavior to use Color.accentColor instead of the prior placeholder.

Technical Notes: macOS accent mapping supports multicolor and hardware accent fallbacks; non-macOS platforms currently default to blue until further system wiring is implemented.

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[augmentcode]

Review completed. 1 suggestion posted.

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[codecov]

Codecov Report

❌ Patch coverage is 3.88889% with 173 lines in your changes missing coverage. Please review.
✅ Project coverage is 26.72%. Comparing base (2c0c72a) to head (4c8a8f1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...e/Graphic/Color/CoreUISystemCatalogColorName.swift	0.00%	52 Missing ⚠️
...es/OpenSwiftUICore/Graphic/Color/AccentColor.swift	8.16%	45 Missing ⚠️
...penSwiftUICore/Graphic/Appearance/Appearance.swift	0.00%	33 Missing ⚠️
...SwiftUI/Graphic/Color/KitAccentColorProvider.swift	0.00%	29 Missing ⚠️
...tUICore/Graphic/Appearance/CatalogAppearance.swift	0.00%	12 Missing ⚠️
...es/OpenSwiftUI/View/Toggle/SwitchToggleStyle.swift	0.00%	1 Missing ⚠️
Sources/OpenSwiftUICore/Shape/Tint.swift	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #887      +/-   ##
==========================================
- Coverage   26.81%   26.72%   -0.10%     
==========================================
  Files         682      686       +4     
  Lines       47622    47796     +174     
==========================================
+ Hits        12771    12773       +2     
- Misses      34851    35023     +172     

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