⚡ Optimize AStar.Solve by reusing state collections

[johnstrand]

🎯 What: Replaced per-solve dictionary, hashset, and priority queue allocations in AStar.Solve with reusable instance fields (_gScore, _previousNodes, _visited, _open).
📊 Coverage: Passed all 410 unit tests in GameUtils.Tests.
✨ Result: Reduced memory allocation per pathfinding operation from 30.32 KB to 5.01 KB (~83.5% allocation reduction) and improved execution time from 27.62 us to 19.39 us (~30% speedup).

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PR created automatically by Jules for task 17565362982342470063 started by @johnstrand

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