Implement VQT Fallback Avatar component for automatic fallback avatar setting

[Copilot]

• [x] Create FallbackAvatar runtime component (marker component)
• [x] Create FallbackAvatarEditor custom inspector
• [x] Add localization strings for the component (EN, JA)
• [x] Implement upload success callback using IVRCSDKPreprocessAvatarCallback and SDK events
• [x] Add IAvatarRootComponent interface for avatar root validation
• [x] Refactor to remove NDMF dependency from callback mechanism
• [x] Refine callback implementation based on code review
• [x] Address additional code review feedback
• [x] Merge master branch
• [x] Address unresolved code review comments
• [ ] Test the implementation

Architecture

Callback Structure

ActualPerformanceCallback (Editor/Callbacks/ActualPerformanceCallback.cs):

• Calculates actual performance rating without NDMF dependency
• Runs with callbackOrder => int.MaxValue (runs last to calculate final performance)
• Stores performance rating in static LastActualPerformanceRating dictionary
• Documented thread safety: Unity main thread only access via synchronization context

FallbackAvatarCallback (Editor/Callbacks/FallbackAvatarCallback.cs):

• Detects FallbackAvatar component with callbackOrder => -100000 (before NDMF optimizations)
• Early returns for PC builds (only processes mobile builds)
• Subscribes to OnSdkUploadSuccess event
• Smart pending avatar cleanup:
  • Keeps in pending list if performance rating not found (allows retry)
  • Removes if performance requirement not met
  • Removes after successful fallback setting
• Uses TryGetValue for efficient dictionary access
• Wrapped in try-catch for safety with documented async void behavior

NDMF Compatibility

• NDMF files (VRCQuestToolsActualPerformanceCallback.cs, NdmfSessionState.cs) remain unchanged
• Callbacks work correctly whether NDMF is installed or not

Recent Updates

• Addressed code review feedback on pending avatar cleanup logic
• Added thread safety documentation
• Improved retry behavior for missing performance ratings

Original prompt

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This section details on the original issue you should resolve

<issue_title>VQT Fallback Avatar コンポーネントの実装</issue_title> <issue_description>親Issue kurotu/VRCQuestTools#153 の要件に従ってコンポーネント及びフォールバック設定の適用処理を作成する。</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes kurotu/VRCQuestTools#164

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