refactor(config): move grid-style Save off the UI thread

Docs/architecture/grid-style-configuration.md

@@ -230,14 +230,19 @@ GridStyleEditorWindow
 
 - **Facade.** `GridStyleEditorFacade` is the only public Core seam for the editor:
   `Load(configDir)` → `Result<GridStyleOptions>`, `Validate(GridStyleOptions)` → `Result`,
-  `Save(configDir, GridStyleOptions)` → `Result`. The loader, validator, writer, and the ~12 DTOs stay
-  `internal`. The UI never touches Core internals directly. (Layering: the config stays in
-  `SemiStep.Core`, settled by review; the editor reaches it only through this facade.)
+  `Save(configDir, GridStyleOptions)` → `Task<Result>`. `Save` is async and runs the file write off the
+  UI thread (mirroring `Load`); the editor's `SaveCommand` awaits it. `Validate` stays synchronous —
+  `RecomputeCanSave` calls it per keystroke, and it is an in-memory pass with no I/O. The loader,
+  validator, writer, and the ~12 DTOs stay `internal`. The UI never touches Core internals directly.
+  (Layering: the config stays in `SemiStep.Core`, settled by review; the editor reaches it only through
+  this facade.)
 - **Writer.** `GridStyleWriter` maps the record back to the DTO (`GridStyleDtoMapper`), serializes with
   the underscored naming convention, re-prepends the file's leading comment block (the header), then
   normalizes to LF and writes UTF-8 no-BOM via a temp-then-move atomic replace in the same `ui/` dir.
-  Every color DTO property carries `[YamlMember(ScalarStyle = ScalarStyle.DoubleQuoted)]` so hex values
-  emit quoted and quoting stays uniform.
+  The temp write and the header read are async (`WriteAllTextAsync` / `ReadAllLinesAsync`); the final
+  `File.Move` stays synchronous — there is no async move, and it is a fast metadata rename inside the
+  same try/catch. Every color DTO property carries `[YamlMember(ScalarStyle = ScalarStyle.DoubleQuoted)]`
+  so hex values emit quoted and quoting stays uniform.
 
 The editor edits the **merged** record (defaults already applied), so `Save` writes a
 fully-populated file — every key is emitted, no omitted-key preservation. That is acceptable for a

SemiStep/SemiStep.Core/Configuration/GridStyleEditorFacade.cs

@@ -38,14 +38,14 @@ public Result Validate(GridStyleOptions options)
 		return GridStyleValidator.Validate(GridStyleDtoMapper.Map(options));
 	}
 
-	public Result Save(string configDir, GridStyleOptions options)
+	public async Task<Result> Save(string configDir, GridStyleOptions options)
 	{
 		var validation = Validate(options);
 		if (validation.IsFailed)
 		{
 			return validation;
 		}
 
-		return _gridStyleWriter.Save(configDir, options);
+		return await _gridStyleWriter.SaveAsync(configDir, options);
 	}
 }

SemiStep/SemiStep.Core/Configuration/IGridStyleEditorFacade.cs

@@ -12,5 +12,5 @@ public interface IGridStyleEditorFacade
 
 	Result Validate(GridStyleOptions options);
 
-	Result Save(string configDir, GridStyleOptions options);
+	Task<Result> Save(string configDir, GridStyleOptions options);
 }

SemiStep/SemiStep.Core/Configuration/Loaders/GridStyleWriter.cs

@@ -17,7 +17,7 @@ internal sealed class GridStyleWriter
 
 	private static readonly UTF8Encoding _utf8NoBom = new(encoderShouldEmitUTF8Identifier: false);
 
-	public Result Save(string configDirectory, GridStyleOptions options)
+	public async Task<Result> SaveAsync(string configDirectory, GridStyleOptions options)
 	{
 		var uiDir = Path.Combine(configDirectory, "ui");
 		var filePath = Path.Combine(uiDir, "grid_style.yaml");
@@ -26,11 +26,11 @@ public Result Save(string configDirectory, GridStyleOptions options)
 		{
 			Directory.CreateDirectory(uiDir);
 
-			var header = ReadLeadingCommentBlock(filePath);
+			var header = await ReadLeadingCommentBlockAsync(filePath);
 			var body = _serializer.Serialize(GridStyleDtoMapper.Map(options));
 			var content = Normalize(header + body);
 
-			WriteAtomic(uiDir, filePath, content);
+			await WriteAtomicAsync(uiDir, filePath, content);
 
 			return Result.Ok();
 		}
@@ -40,14 +40,14 @@ public Result Save(string configDirectory, GridStyleOptions options)
 		}
 	}
 
-	private static string ReadLeadingCommentBlock(string filePath)
+	private static async Task<string> ReadLeadingCommentBlockAsync(string filePath)
 	{
 		if (!File.Exists(filePath))
 		{
 			return string.Empty;
 		}
 
-		var lines = File.ReadAllLines(filePath);
+		var lines = await File.ReadAllLinesAsync(filePath);
 		var headerLines = new List<string>();
 
 		foreach (var line in lines)
@@ -76,11 +76,11 @@ private static string Normalize(string content)
 		return content.Replace("\r\n", "\n").Replace("\r", "\n");
 	}
 
-	private static void WriteAtomic(string uiDir, string filePath, string content)
+	private static async Task WriteAtomicAsync(string uiDir, string filePath, string content)
 	{
 		var tempPath = Path.Combine(uiDir, $".grid_style.{Guid.NewGuid():N}.tmp");
 
-		File.WriteAllText(tempPath, content, _utf8NoBom);
+		await File.WriteAllTextAsync(tempPath, content, _utf8NoBom);
 
 		try
 		{

SemiStep/SemiStep.Tests/Core/Configuration/GridStyleEditorFacadeTests.cs

@@ -48,7 +48,7 @@ public async Task SaveThenLoad_RoundTrips()
 
 		var original = (await facade.Load(tempDir.Path)).Value;
 
-		facade.Save(tempDir.Path, original).IsSuccess.Should().BeTrue();
+		(await facade.Save(tempDir.Path, original)).IsSuccess.Should().BeTrue();
 
 		var reloaded = (await facade.Load(tempDir.Path)).Value;
 
@@ -61,7 +61,7 @@ public async Task SaveThenLoad_DistinctFixture_PreservesEveryMappedField()
 		using var tempDir = new TempDirectory();
 		var facade = new GridStyleEditorFacade();
 
-		facade.Save(tempDir.Path, GridStyleOptionsTestData.Distinct()).IsSuccess.Should().BeTrue();
+		(await facade.Save(tempDir.Path, GridStyleOptionsTestData.Distinct())).IsSuccess.Should().BeTrue();
 
 		var reloaded = (await facade.Load(tempDir.Path)).Value;
 
@@ -101,7 +101,7 @@ public async Task Save_InvalidRecord_FailsValidationGuardBeforeWriting()
 			with
 		{ ExecutionDepth0Color = "not-a-color" };
 
-		facade.Save(tempDir.Path, invalid).IsFailed.Should().BeTrue();
+		(await facade.Save(tempDir.Path, invalid)).IsFailed.Should().BeTrue();
 
 		var after = await File.ReadAllTextAsync(filePath, token);
 		after.Should().Be(before);
@@ -128,13 +128,13 @@ await File.WriteAllTextAsync(
 	}
 
 	[Fact]
-	public void Save_WriteFailure_CarriesOriginalExceptionOnCausedBy()
+	public async Task Save_WriteFailure_CarriesOriginalExceptionOnCausedBy()
 	{
 		using var tempDir = new TempDirectory();
 		var configDirAsFile = Path.Combine(tempDir.Path, "not-a-directory");
 		File.WriteAllText(configDirAsFile, string.Empty);
 
-		var result = new GridStyleWriter().Save(configDirAsFile, GridStyleOptions.Default);
+		var result = await new GridStyleWriter().SaveAsync(configDirAsFile, GridStyleOptions.Default);
 
 		result.IsFailed.Should().BeTrue();
 		result.Errors.Should().ContainSingle().Which.Should().BeOfType<GridStyleSaveFailedError>();

SemiStep/SemiStep.Tests/Core/Configuration/GridStyleOrientationTests.cs

@@ -82,7 +82,7 @@ public async Task SaveRoundTrip_PreservesOrientation()
 		await AppendOrientation(tempDir.Path, "columns_as_steps");
 		var original = await LoadValidated(tempDir.Path);
 
-		new GridStyleWriter().Save(tempDir.Path, original).IsSuccess.Should().BeTrue();
+		(await new GridStyleWriter().SaveAsync(tempDir.Path, original)).IsSuccess.Should().BeTrue();
 
 		var reloaded = await LoadValidated(tempDir.Path);
 		reloaded.Orientation.Should().Be(GridOrientation.ColumnsAsSteps);
@@ -95,7 +95,7 @@ public async Task Save_MissingOrientation_EmitsExplicitDefault()
 		using var tempDir = CopyShippedConfig("MOCVD");
 		var options = await LoadValidated(tempDir.Path);
 
-		new GridStyleWriter().Save(tempDir.Path, options).IsSuccess.Should().BeTrue();
+		(await new GridStyleWriter().SaveAsync(tempDir.Path, options)).IsSuccess.Should().BeTrue();
 
 		var content = await File.ReadAllTextAsync(
 			Path.Combine(tempDir.Path, "ui", "grid_style.yaml"),

SemiStep/SemiStep.Tests/Core/Configuration/GridStyleWriterTests.cs

@@ -30,7 +30,7 @@ public async Task Save_SemanticRoundTrip_PreservesRecord(string equipment)
 
 		var original = await LoadOptions(tempDir.Path);
 
-		new GridStyleWriter().Save(tempDir.Path, original).IsSuccess.Should().BeTrue();
+		(await new GridStyleWriter().SaveAsync(tempDir.Path, original)).IsSuccess.Should().BeTrue();
 
 		var reloaded = await LoadOptions(tempDir.Path);
 
@@ -50,7 +50,7 @@ public async Task Save_PreservesLeadingCommentBlock_WhenHeaderPresent(string equ
 		originalHeader.Should().NotBeEmpty();
 		var options = await LoadOptions(tempDir.Path);
 
-		new GridStyleWriter().Save(tempDir.Path, options).IsSuccess.Should().BeTrue();
+		(await new GridStyleWriter().SaveAsync(tempDir.Path, options)).IsSuccess.Should().BeTrue();
 
 		var writtenHeader = LeadingCommentBlock(await File.ReadAllTextAsync(filePath, token));
 		writtenHeader.Should().Be(originalHeader);
@@ -67,7 +67,7 @@ public async Task Save_InjectsNoHeader_WhenSourceHasNone()
 		LeadingCommentBlock(await File.ReadAllTextAsync(filePath, token)).Should().BeEmpty();
 		var options = await LoadOptions(tempDir.Path);
 
-		new GridStyleWriter().Save(tempDir.Path, options).IsSuccess.Should().BeTrue();
+		(await new GridStyleWriter().SaveAsync(tempDir.Path, options)).IsSuccess.Should().BeTrue();
 
 		var written = (await File.ReadAllTextAsync(filePath, token)).Replace("\r\n", "\n");
 		var firstNonBlank = written.Split('\n').First(line => line.TrimStart().Length > 0);
@@ -81,7 +81,7 @@ public async Task Save_EmitsColorScalarsDoubleQuoted()
 		var filePath = GridStyleFilePath(tempDir.Path);
 		var options = await LoadOptions(tempDir.Path);
 
-		new GridStyleWriter().Save(tempDir.Path, options).IsSuccess.Should().BeTrue();
+		(await new GridStyleWriter().SaveAsync(tempDir.Path, options)).IsSuccess.Should().BeTrue();
 
 		var content = await File.ReadAllTextAsync(filePath, TestContext.Current.CancellationToken);
 		content.Should().Contain("\"#");
@@ -109,7 +109,7 @@ public async Task Save_WhenMoveFails_FailsAndLeavesOriginalIntactWithNoTempOrpha
 		// Hold an exclusive lock on the target so the atomic File.Move cannot replace it.
 		using (var locking = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.None))
 		{
-			result = new GridStyleWriter().Save(tempDir.Path, options);
+			result = await new GridStyleWriter().SaveAsync(tempDir.Path, options);
 		}
 
 		result.IsFailed.Should().BeTrue();
@@ -127,7 +127,7 @@ public async Task Save_WritesLfLineEndingsWithoutBom()
 		var filePath = GridStyleFilePath(tempDir.Path);
 		var options = await LoadOptions(tempDir.Path);
 
-		new GridStyleWriter().Save(tempDir.Path, options).IsSuccess.Should().BeTrue();
+		(await new GridStyleWriter().SaveAsync(tempDir.Path, options)).IsSuccess.Should().BeTrue();
 
 		var bytes = await File.ReadAllBytesAsync(filePath, TestContext.Current.CancellationToken);
 		bytes.Take(3).Should().NotEqual(new byte[] { 0xEF, 0xBB, 0xBF });
@@ -145,7 +145,7 @@ public async Task Save_UnderCommaLocale_EmitsInvariantDecimals()
 		try
 		{
 			CultureInfo.CurrentCulture = new CultureInfo("de-DE");
-			new GridStyleWriter().Save(tempDir.Path, options).IsSuccess.Should().BeTrue();
+			(await new GridStyleWriter().SaveAsync(tempDir.Path, options)).IsSuccess.Should().BeTrue();
 		}
 		finally
 		{

SemiStep/SemiStep.Tests/UI/StyleEditor/GridStyleEditorViewModelTests.cs

@@ -509,7 +509,7 @@ public Result Validate(GridStyleOptions options)
 			return Result.Ok();
 		}
 
-		public Result Save(string configDir, GridStyleOptions options)
+		public Task<Result> Save(string configDir, GridStyleOptions options)
 		{
 			throw failure;
 		}
@@ -529,9 +529,9 @@ public Result Validate(GridStyleOptions options)
 			return Result.Ok();
 		}
 
-		public Result Save(string configDir, GridStyleOptions options)
+		public Task<Result> Save(string configDir, GridStyleOptions options)
 		{
-			return saveResult ?? Result.Ok();
+			return Task.FromResult(saveResult ?? Result.Ok());
 		}
 	}
 }

SemiStep/SemiStep.UI/StyleEditor/GridStyleEditorViewModel.cs

@@ -62,8 +62,8 @@ public GridStyleEditorViewModel(
 		_source = source;
 		_logger = logger;
 
-		SaveCommand = ReactiveCommand.Create(
-			Save,
+		SaveCommand = ReactiveCommand.CreateFromTask(
+			SaveAsync,
 			this.WhenAnyValue(viewModel => viewModel.CanSave));
 
 		// Modal editor: a save fault must surface on the editor's own ErrorMessage, not the
@@ -284,15 +284,15 @@ public GridStyleOptions BuildRecord()
 		};
 	}
 
-	private bool Save()
+	private async Task<bool> SaveAsync()
 	{
 		if (!CanSave)
 		{
 			ErrorMessage = Resources.EditorCannotSave;
 			return false;
 		}
 
-		var result = _gridStyleEditorFacade.Save(_configDir, BuildRecord());
+		var result = await _gridStyleEditorFacade.Save(_configDir, BuildRecord());
 		if (result.IsFailed)
 		{
 			ErrorMessage = string.Join("; ", result.Errors.Select(ReasonLocalizer.Localize));