⚡ Optimize MapMagicBrush lookup in Undo event loops

[marko1olo]

💡 What: Implemented a static HashSet cache for MapMagicBrush instances. It is populated securely on the main thread via Awake() and OnDestroy(), ensuring no thread-safety issues during Unity domain reloads. The expensive FindObjectsByType search query in UndoBac.cs and UndoFull.cs has been replaced by simply iterating over this cached registry.

🎯 Why: Scene-wide object search (GameObject.FindObjectsByType) with FindObjectsInactive.Include is exceptionally slow. It causes a C++-to-C# transition, allocates garbage, and searches the entire scene hierarchy on every brush undo/redo event loop, causing major frame time drops.

📊 Measured Improvement: Replaces an O(N) operation (where N = number of GameObjects in the scene hierarchy) with an O(M) operation (where M = total active/instantiated brushes in the cache, usually <= 1) and completely skips hierarchical layout searches.

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PR created automatically by Jules for task 11687578998825631762 started by @marko1olo

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