⚡ Bolt: Eliminate dynamic grouping allocations in RobotMerger

[lordhippo]

💡 What:
Replaced per-frame SelectMany, GroupBy, ToDictionary, and ToList in RobotMerger.Process with explicitly indexed pre-allocated arrays and lists. Handled unknown or invalid Robot IDs via a separate struct-keyed dictionary + object pool.

🎯 Why:
The LINQ methods allocated enumerators, IGrouping instances, closures, dictionaries, and new lists at ~100Hz, leading to heavy GC pressure. The fixed robot count (CommonConfigs.MaxRobots * 2) allows for an O(1) allocation-free mapping using arrays instead of hash tables.

📊 Impact:
Eliminates all per-frame grouping and collection allocations inside RobotMerger.Process (savings scaling linearly with active tracker counts on each camera).

🔬 Measurement:
dotnet-counters monitor --counters System.Runtime[gen-0-gc-count,alloc-rate]

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PR created automatically by Jules for task 10968147264723719111 started by @lordhippo

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