Refactor benchmarks to use typed enums for engine/mode
Replaced string-based identifiers for serializer engine, I/O mode, and dispatch mode with strongly-typed enums (BenchmarkEngine, BenchmarkIoMode, BenchmarkDispatchMode). Added BenchmarkEnums.cs with ToDisplay() helpers for consistent output. Updated all benchmark implementations, DTOs, and output logic to use enums. Removed obsolete string constants from Configuration.cs. Merged allocation measurement methods in BenchmarkLoop.cs for clarity. Improves type safety, maintainability, and output consistency.
Moved all ISerializerBenchmark implementations for AcBinary and MemoryPack from Program.cs into dedicated files under Benchmarks/. Improves code organization and maintainability; no logic changes, only file structure refactor.
Loretta