Add non-resizable memory implementations

[konard]

Summary

This PR adds non-resizable (fixed-size) memory implementations to complement the existing resizable memory implementations. These are particularly useful for scenarios where you want to allocate all available resources at once (e.g., all RAM or a whole drive) and avoid time wasted on reallocations.

New Components Added:

• IFixedDirectMemory - Interface for fixed-size direct memory access that extends IDirectMemory
• FixedDirectMemoryBase - Abstract base class providing common functionality for fixed memory implementations
• HeapFixedDirectMemory - Fixed-size memory allocation using unmanaged heap memory
• FileMappedFixedDirectMemory - Fixed-size memory allocation using memory-mapped files
• Comprehensive unit tests - Full test coverage for all new implementations

Key Features:

• Performance optimized - No resize overhead since size is fixed at allocation
• Resource allocation - Ideal for allocating all available RAM or storage at once
• Consistent API - Follows the same patterns as existing resizable memory implementations
• Memory safety - Proper disposal and error handling
• Cross-platform - Works on all platforms supported by .NET

Implementation Details:

• All implementations properly handle minimum capacity enforcement
• Memory is zero-initialized for heap allocations
• File-mapped memory supports persistence across application restarts
• Proper disposal prevents memory leaks
• Thread-safe disposal using atomic operations

Test Coverage:

• Constructor validation and capacity enforcement
• Memory allocation and deallocation
• Read/write operations
• Persistence testing for file-mapped memory
• Disposal and error condition testing

Resolves #48

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