Refactor bitmap viewer with proper DI and multi-mode support

[Copilot]

Addresses code review feedback and feature requests for the memory bitmap viewer: improper dependency injection pattern, missing display mode options, and incorrect exception constructors.

Dependency Injection Refactoring

Replaced messenger-based factory pattern with proper constructor injection:

// Before: side-stepping DI with messenger
messenger.Register<CreateMemoryBitmapViewModelMessage>(this, (_, m) => {
    var vm = new MemoryBitmapViewModel(videoState, hostStorageProvider);
    m.SetInstance(vm);
});

// After: proper factory pattern
public interface IMemoryBitmapViewModelFactory {
    MemoryBitmapViewModel CreateNew();
}

public class MemoryBitmapViewModelFactory : IMemoryBitmapViewModelFactory {
    private readonly IVideoState _videoState;
    private readonly IHostStorageProvider _storage;
    // ... constructor injection
}

• Created IMemoryBitmapViewModelFactory with concrete implementation
• Updated all MemoryViewModel variants to accept factory via constructor
• Removed CreateMemoryBitmapViewModelMessage message class

Display Mode Support

Added MemoryBitmapDisplayMode enum with 5 rendering modes:

• VGA 8BPP: 256-color indexed (existing implementation)
• CGA 4-color: 2 bits per pixel with 4-color palette
• EGA 16-color: 4 bits per pixel with 16-color palette
• Text Mode: Character/attribute pairs with IBM PC fonts (placeholder pattern currently)
• Hercules Monochrome: 1 bit per pixel for 720×348 graphics

UI updated with ComboBox for mode selection and CheckBox for optional overlay grid (stub for future implementation).

Exception Constructor Fixes

Corrected 4 instances of ArgumentOutOfRangeException using single-parameter constructor:

// Before
throw new ArgumentOutOfRangeException($"Invalid bitmap size: {width}x{height}");

// After
throw new ArgumentOutOfRangeException(nameof(BitmapViewWidth), $"Invalid bitmap size: {width}x{height}");

Remaining Work

• Text mode needs actual IBM font glyph rendering from video memory plane 2
• Overlay grid implementation for memory address/value visualization
• AvaloniaHex integration for switching between hex and bitmap views

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