feat(theme): apply a theme saved by any process

CLAUDE.md

@@ -33,7 +33,8 @@ The settings window (`SemiPlot.UI/Settings`) is the viewer's only writer of the
 edits only keys that already exist, each in the file that owns it, and only the keys the operator
 changed; it creates no file, and the startup-failure window offers it only when both sections read and carry every key it edits. The
 production loaders validate a staged copy before any file is replaced, and every change takes effect at the
-next start (`docs/architecture/overview.md#the-settings-window`).
+next start except the theme, which every running process applies live
+(`docs/architecture/overview.md#the-live-theme`).
 
 `.editorconfig`'s style and quality analyzer rules fail `dotnet build` (`TreatWarningsAsErrors`,
 `EnforceCodeStyleInBuild`) and `dotnet format SemiPlot.slnx --verify-no-changes` alike, so a

SemiPlot/SemiPlot.Core/Configuration/ConfigurationSection.cs

@@ -128,7 +128,7 @@ private static Dictionary<string, string> ScalarsOf(Dictionary<string, object?>
 	{
 		try
 		{
-			var content = File.ReadAllText(file);
+			var content = ReadShared(file);
 			var repeated = FindRepeatedKey(content);
 
 			if (repeated is not null)
@@ -146,6 +146,20 @@ private static Dictionary<string, string> ScalarsOf(Dictionary<string, object?>
 		}
 	}
 
+	private static string ReadShared(string file)
+	{
+		using var stream = OpenShared(file);
+		using var reader = new StreamReader(stream);
+
+		return reader.ReadToEnd();
+	}
+
+	/// <summary>Opens a section file for reading without blocking another process's File.Replace of it.</summary>
+	internal static FileStream OpenShared(string file)
+	{
+		return new FileStream(file, FileMode.Open, FileAccess.Read, FileShare.ReadWrite | FileShare.Delete);
+	}
+
 	// WithDuplicateKeyChecking refuses the file but names the key only inside its own message, and the
 	// operator needs the key, so the top-level mapping is walked for it first.
 	private static string? FindRepeatedKey(string content)

SemiPlot/SemiPlot.Core/Configuration/ConfigurationSectionWriter.cs

@@ -163,7 +163,7 @@ private Result Copy(string file, string name)
 
 			try
 			{
-				content = File.ReadAllBytes(file);
+				content = ReadAllBytesShared(file);
 			}
 			catch (Exception exception)
 			{
@@ -184,6 +184,15 @@ private Result Copy(string file, string name)
 			}
 		}
 
+		private static byte[] ReadAllBytesShared(string file)
+		{
+			using var stream = ConfigurationSection.OpenShared(file);
+			using var content = new MemoryStream();
+			stream.CopyTo(content);
+
+			return content.ToArray();
+		}
+
 		private Result Write(string name, string content)
 		{
 			try

SemiPlot/SemiPlot.Tests.Unit/Core/Configuration/ConfigurationSectionTests.cs

@@ -145,6 +145,37 @@ public void AnEmptyFileContributesNothingAndFailsNothing()
 			.Be(new KeyValuePair<string, string>("locale", "ru"));
 	}
 
+	[Fact]
+	public void AFileAnotherHandleHoldsOpenForWritingStillReads()
+	{
+		WriteFile("a.yaml", "locale: ru\n");
+		using var writer = new FileStream(
+			Path.Combine(_directory, "a.yaml"), FileMode.Open, FileAccess.ReadWrite, FileShare.Read);
+
+		var result = ConfigurationSection.Read(_directory, ConfigurationSectionName.App);
+
+		result.IsSuccess.Should().BeTrue(Describe(result));
+		MappingOf(result.Value).Should().Contain("locale", "ru");
+	}
+
+	[Fact]
+	public void AFileTheSectionReadHoldsOpenCanStillBeReplaced()
+	{
+		WriteFile("a.yaml", "locale: ru\n");
+		var staged = Path.Combine(_directory, "a.yaml.staged");
+		File.WriteAllText(staged, "locale: en\n");
+
+		using (ConfigurationSection.OpenShared(Path.Combine(_directory, "a.yaml")))
+		{
+			File.Replace(staged, Path.Combine(_directory, "a.yaml"), destinationBackupFileName: null);
+		}
+
+		var result = ConfigurationSection.Read(_directory, ConfigurationSectionName.App);
+
+		result.IsSuccess.Should().BeTrue(Describe(result));
+		MappingOf(result.Value).Should().Contain("locale", "en");
+	}
+
 	[Fact]
 	public void AFileWrittenWithAByteOrderMarkStillContributesItsFirstKey()
 	{

SemiPlot/SemiPlot.Tests.Unit/Core/Configuration/ConfigurationSectionWriterTests.cs

@@ -188,6 +188,24 @@ public void ASectionFolderRemovedAfterTheReadFailsWithUnlistable()
 		error.Directory.Should().Be(_section);
 	}
 
+	[Fact]
+	public void AFileAnotherSavesWriteProbeHoldsOpenIsStillCopied()
+	{
+		WriteFile("a.yaml", "locale: ru\n");
+		WriteFile("b.yaml", "theme: light\n");
+		var owned = OwnedOf(ConfigurationSectionName.App);
+		var held = Path.Combine(_section, "b.yaml");
+		Result<IReadOnlyList<string>> staged;
+
+		using (new FileStream(held, FileMode.Open, FileAccess.Write, FileShare.ReadWrite | FileShare.Delete))
+		{
+			staged = Stage(owned, ConfigurationSectionName.App, ("locale", "en"));
+		}
+
+		staged.IsSuccess.Should().BeTrue(Describe(staged));
+		File.ReadAllBytes(Path.Combine(_staging, "b.yaml")).Should().Equal(File.ReadAllBytes(held));
+	}
+
 	[Fact]
 	public void ACopyThatCannotBeWrittenFailsWithUnwritableNamingTheFile()
 	{