Modernize Asset Browser

[freezy]

This PR modernizes the Unity Editor asset browser:

• fixes query parsing, lifecycle handling, serialization, missing-reference handling, path handling, and search debouncing
• virtualizes the asset grid with pooled ListView rows while preserving cell selection behavior
• loads thumbnails asynchronously with cancellation, bounded concurrency, and a size-aware LRU cache
• adds indexed queries, tokenizer-based parsing, and automatic index invalidation after asset changes

Notes

The implementation uses ListView row virtualization because UI Toolkit does not provide a native virtualized grid, and retains custom per-cell selection semantics.

[greptile-apps]

Greptile Summary

This PR modernizes the Unity Editor asset browser. The main changes are:

• Virtualized asset grid rows with pooled cells.
• Async thumbnail loading with cancellation and cache limits.
• Indexed asset search with tokenizer-based query parsing.
• Debounced search and library refresh handling.
• Asset-change invalidation for library indexes and visible results.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
VisualPinball.Unity/VisualPinball.Unity.Editor/AssetBrowser/AssetBrowser.cs	Adds library-change scheduling, result refresh after reindexing, path matching updates, and virtualized grid state handling.
VisualPinball.Unity/VisualPinball.Unity.Editor/AssetBrowser/AssetBrowserPostprocessor.cs	Adds editor asset-change notifications for relevant asset browser files.
VisualPinball.Unity/VisualPinball.Unity.Editor/AssetBrowser/AssetQuery.cs	Moves searches onto per-library indexes and supports explicit library reindexing.

_{Reviews (2): Last reviewed commit: "editor: refresh asset results after rein..." | Re-trigger Greptile}