refactor(int64): create minimal ThrowHelper and delete unused files

Created UnmanagedSpanThrowHelper.cs with minimal exception helpers:
- ThrowArgumentOutOfRangeException
- ThrowArgumentNullException
- ThrowArgumentException_DestinationTooShort
- ThrowArrayTypeMismatchException
- ThrowIndexOutOfRangeException
- ThrowInvalidOperationException
- ThrowArgument_TypeContainsReferences
- SR string constants for error messages

Deleted 30 files that duplicate .NET built-in functionality:
- Unsafe.cs, RuntimeHelpers.cs (use System.Runtime.CompilerServices)
- MemoryMarshal*.cs (use System.Runtime.InteropServices)
- Index.cs, Range.cs (use System.Index, System.Range)
- Vector*.cs, BitOperations.cs (use System.Runtime.Intrinsics)
- Memory.cs, ReadOnlyMemory.cs (use System.Memory<T>)
- *Pool.cs, *Manager.cs, *Handle.cs (buffer infrastructure)
- *Sequence*.cs, *BufferWriter*.cs (not needed)
- *Marshaller.cs, *Enumerator.cs (P/Invoke, text - not needed)
- SearchValues.cs, Marvin.cs, NativeMemory.cs (not needed)
- Attribute files, System.*.cs ref assemblies

Remaining 17 files are the core UnmanagedSpan implementation.

Author: Nucs