Add ArrayMemory to DirectMemory adapter implementation (Fixes #22)

[konard]

Summary

This PR implements a solution for issue #22 by providing adapters that convert IArrayMemory<TElement> into IDirectMemory. The implementation uses GCHandle.Alloc() with GCHandleType.Pinned to pin managed arrays in memory and provide stable pointers for direct memory access.

Key Features:

• ArrayMemoryAsDirectMemoryAdapter: Optimized adapter specifically for ArrayMemory<TElement> that uses reflection to access the internal array and pin it directly
• ArrayMemoryAsDirectMemoryAdapterGeneral: General-purpose adapter that works with any IArrayMemory<TElement> implementation by creating a temporary array copy and pinning it
• Comprehensive memory safety: Both adapters properly manage GCHandle lifecycle and provide stable pointer access
• Full test coverage: Unit tests verify functionality, pointer stability, memory safety, and proper dispose behavior

Technical Approach:

The solution leverages the .NET GC pinning mechanism referenced in the issue comments:

• Uses GCHandle.Alloc(array, GCHandleType.Pinned) to prevent GC from moving the array
• Implements IDisposable pattern to ensure proper cleanup of pinned handles
• Provides stable IntPtr access through the IDirectMemory.Pointer property

Usage Examples:

// Using the optimized adapter for ArrayMemory
var arrayMemory = new ArrayMemory<int>(100);
using var directAdapter = new ArrayMemoryAsDirectMemoryAdapter<int>(arrayMemory);
var pointer = directAdapter.Pointer; // Stable pointer for direct access

// Using the general adapter for any IArrayMemory implementation  
var fileArrayMemory = new FileArrayMemory<byte>("data.bin");
using var generalAdapter = new ArrayMemoryAsDirectMemoryAdapterGeneral<byte>(fileArrayMemory);
var filePointer = generalAdapter.Pointer; // Works with file-backed memory too

Test Plan

• ✅ All existing tests pass
• ✅ New comprehensive test suite covering both adapters
• ✅ Memory safety tests verify proper GC handle management
• ✅ Pointer stability tests ensure pinning works correctly
• ✅ Disposal tests verify proper cleanup
• ✅ Cross-implementation tests (works with FileArrayMemory, etc.)
• ✅ Build passes in both Debug and Release configurations

🤖 Generated with Claude Code

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Resolves #22