⚡ Reuse internal collections in Dijkstra.Solve

[johnstrand]

💡 What:
Reused internal instance collections (_distances, _previousNodes, _visited, and _queue) across calls to Dijkstra<T>.Solve instead of allocating new Dictionary, HashSet, and PriorityQueue objects on every pathfinding query.

🎯 Why:
Instantiating new tracking data structures on every Solve call causes unnecessary per-call GC heap allocations and overhead during shortest-path calculations.

📊 Measured Improvement:
BenchmarkDotNet results for DijkstraBenchmarks.SolveBenchmark:

• Allocated Memory: Reduced from 31.55 KB to 4.31 KB per operation (-86.3% allocation reduction).
• Execution Time (Mean): Reduced from 34.16 us to 25.12 us (-26.5% speedup).
• GC Gen0 Collections: Reduced from 1.3428 to 0.1831 per 1,000 operations (-86.4%).

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PR created automatically by Jules for task 13125347420987556613 started by @johnstrand

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