Refactor: move benchmark loop logic to BenchmarkLoop.cs
Refactored all benchmark execution infrastructure from Program.cs into a new internal static class BenchmarkLoop. This includes timing, allocation measurement, progress reporting, GC helpers, MemoryPack setup validation, and test data filtering. Updated Program.cs and all serializer benchmarks to use the new class. Added serAllocPct reporting in Output.cs and a PowerShell script for automated refactoring. No functional changes to benchmark logic.
