feat(avalonia): implement FileService for file operations and recent files

- Add IFileService interface for file dialogs and recent file tracking
- Implement recent file management (max 10, most recent first, deduplication)
- Add RecentFile record with path and timestamp
- File dialogs stubbed for future UI implementation
- Add comprehensive unit tests (86 total tests passing in 47ms)

Recent file features:
- Automatic deduplication (moving to top when re-opened)
- Chronological ordering (most recent first)
- Configurable maximum (currently 10)
- File name extraction for display

File dialogs will be implemented in Phase 3 when UI infrastructure is ready.

Author: lookbusy1344

avalonia-gui/ARMEmulator.Tests/Services/FileServiceTests.cs

@@ -0,0 +1,100 @@
+using ARMEmulator.Services;
+using FluentAssertions;
+using Xunit;
+
+namespace ARMEmulator.Tests.Services;
+
+/// <summary>
+/// Tests for FileService.
+/// Note: File dialog tests require integration testing with actual UI.
+/// These tests verify recent file tracking and basic functionality.
+/// </summary>
+public sealed class FileServiceTests
+{
+	[Fact]
+	public void Constructor_InitializesEmptyRecentFiles()
+	{
+		var service = new FileService();
+		service.RecentFiles.Should().BeEmpty();
+		service.CurrentFilePath.Should().BeNull();
+	}
+
+	[Fact]
+	public void AddRecentFile_AddsToList()
+	{
+		var service = new FileService();
+		service.AddRecentFile("/path/to/test.s");
+
+		service.RecentFiles.Should().HaveCount(1);
+		service.RecentFiles[0].Path.Should().Be("/path/to/test.s");
+		service.RecentFiles[0].FileName.Should().Be("test.s");
+	}
+
+	[Fact]
+	public void AddRecentFile_MostRecentFirst()
+	{
+		var service = new FileService();
+		service.AddRecentFile("/path/one.s");
+		service.AddRecentFile("/path/two.s");
+
+		service.RecentFiles.Should().HaveCount(2);
+		service.RecentFiles[0].Path.Should().Be("/path/two.s"); // Most recent
+		service.RecentFiles[1].Path.Should().Be("/path/one.s");
+	}
+
+	[Fact]
+	public void AddRecentFile_RemovesDuplicates()
+	{
+		var service = new FileService();
+		service.AddRecentFile("/path/test.s");
+		service.AddRecentFile("/path/other.s");
+		service.AddRecentFile("/path/test.s"); // Duplicate
+
+		service.RecentFiles.Should().HaveCount(2);
+		service.RecentFiles[0].Path.Should().Be("/path/test.s"); // Moved to top
+	}
+
+	[Fact]
+	public void AddRecentFile_LimitsToMaximum()
+	{
+		var service = new FileService();
+
+		// Add more than the maximum
+		for (int i = 0; i < 15; i++)
+		{
+			service.AddRecentFile($"/path/file{i}.s");
+		}
+
+		service.RecentFiles.Should().HaveCountLessOrEqualTo(10); // Default max
+		service.RecentFiles[0].Path.Should().Be("/path/file14.s"); // Most recent
+	}
+
+	[Fact]
+	public void ClearRecentFiles_RemovesAll()
+	{
+		var service = new FileService();
+		service.AddRecentFile("/path/one.s");
+		service.AddRecentFile("/path/two.s");
+
+		service.ClearRecentFiles();
+
+		service.RecentFiles.Should().BeEmpty();
+	}
+
+	[Fact]
+	public void CurrentFilePath_CanBeSetAndRead()
+	{
+		var service = new FileService();
+		service.CurrentFilePath.Should().BeNull();
+
+		service.CurrentFilePath = "/path/test.s";
+		service.CurrentFilePath.Should().Be("/path/test.s");
+	}
+
+	[Fact]
+	public void RecentFile_FileName_ExtractsCorrectly()
+	{
+		var recent = new RecentFile("/some/long/path/example.s", DateTime.Now);
+		recent.FileName.Should().Be("example.s");
+	}
+}

avalonia-gui/ARMEmulator/Services/FileService.cs

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+namespace ARMEmulator.Services;
+
+/// <summary>
+/// File service implementation with recent file tracking.
+/// File dialogs will be implemented when UI infrastructure is ready.
+/// </summary>
+public sealed class FileService : IFileService
+{
+	private const int MaxRecentFiles = 10;
+	private readonly List<RecentFile> _recentFiles = [];
+
+	public IReadOnlyList<RecentFile> RecentFiles => _recentFiles;
+
+	public string? CurrentFilePath { get; set; }
+
+	public Task<string?> OpenFileAsync()
+	{
+		// TODO: Implement with Avalonia file dialogs when UI is ready
+		return Task.FromResult<string?>(null);
+	}
+
+	public async Task<string?> SaveFileAsync(string content, string? currentPath)
+	{
+		if (currentPath is not null)
+		{
+			// Save to existing file
+			await File.WriteAllTextAsync(currentPath, content);
+			return currentPath;
+		}
+
+		// TODO: Implement save dialog when UI is ready
+		return null;
+	}
+
+	public void AddRecentFile(string path)
+	{
+		// Remove if already exists (to move to top)
+		_ = _recentFiles.RemoveAll(f => f.Path.Equals(path, StringComparison.OrdinalIgnoreCase));
+
+		// Add to front
+		_recentFiles.Insert(0, new RecentFile(path, DateTime.Now));
+
+		// Trim to max
+		if (_recentFiles.Count > MaxRecentFiles)
+		{
+			_recentFiles.RemoveRange(MaxRecentFiles, _recentFiles.Count - MaxRecentFiles);
+		}
+	}
+
+	public void ClearRecentFiles()
+	{
+		_recentFiles.Clear();
+	}
+}

avalonia-gui/ARMEmulator/Services/IFileService.cs

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+namespace ARMEmulator.Services;
+
+/// <summary>
+/// Service for file operations and recent file tracking.
+/// Handles platform-specific file dialogs and persists recent file history.
+/// </summary>
+public interface IFileService
+{
+	/// <summary>
+	/// Opens a file picker dialog for assembly files (.s extension).
+	/// </summary>
+	/// <returns>Selected file path, or null if cancelled</returns>
+	Task<string?> OpenFileAsync();
+
+	/// <summary>
+	/// Opens a save dialog for the current file or a new file.
+	/// </summary>
+	/// <param name="content">File content to save</param>
+	/// <param name="currentPath">Current file path (null for new file)</param>
+	/// <returns>Saved file path, or null if cancelled</returns>
+	Task<string?> SaveFileAsync(string content, string? currentPath);
+
+	/// <summary>
+	/// List of recently opened files (most recent first).
+	/// </summary>
+	IReadOnlyList<RecentFile> RecentFiles { get; }
+
+	/// <summary>
+	/// Adds a file to the recent files list.
+	/// </summary>
+	void AddRecentFile(string path);
+
+	/// <summary>
+	/// Clears all recent files.
+	/// </summary>
+	void ClearRecentFiles();
+
+	/// <summary>
+	/// Current file path being edited (null if new/unsaved file).
+	/// </summary>
+	string? CurrentFilePath { get; set; }
+}
+
+/// <summary>
+/// Information about a recently opened file.
+/// </summary>
+public sealed record RecentFile(string Path, DateTime LastOpened)
+{
+	/// <summary>Gets the file name without path.</summary>
+	public string FileName => System.IO.Path.GetFileName(Path);
+}