Document the two roles of the language version in the settings API

The language version appears in two places that share a name but not a
concept, which repeatedly reads as one confused API: on
DecompilerSettings it is a construction shortcut (SetLanguageVersion
initializes the feature flags once and the version is not stored, so
the flags are the only state and the call is deliberately one-way),
while on WholeProjectDecompiler it is an export parameter (the
LangVersion stamped into the project file, defaulting to
GetMinimumRequiredVersion() and rejected below it as a safety net
against exporting uncompilable projects). Spell both roles out in the
XML docs so the distinction no longer has to be reverse-engineered.

Assisted-by: Claude:claude-fable-5:Claude Code

ICSharpCode.Decompiler.Generators/DecompilerSettingsGenerator.cs

@@ -332,12 +332,12 @@ namespace ICSharpCode.Decompiler
 		context.AddSource(hintName, SourceText.From(builder.ToString().Replace("\r\n", "\n"), Encoding.UTF8));
 	}
 
+	// Emitted as partial implementing declarations: the containing class supplies the defining
+	// stubs, which is where the XML documentation lives (the docs on a partial method's defining
+	// declaration apply as long as the implementation carries none).
 	static void WriteSetLanguageVersion(StringBuilder builder, IGrouping<int, SettingInfo>[] versionBuckets)
 	{
-		builder.AppendLine("\t\t/// <summary>");
-		builder.AppendLine("\t\t/// Deactivates all language features from versions newer than <paramref name=\"languageVersion\"/>.");
-		builder.AppendLine("\t\t/// </summary>");
-		builder.AppendLine("\t\tpublic void SetLanguageVersion(global::ICSharpCode.Decompiler.CSharp.LanguageVersion languageVersion)");
+		builder.AppendLine("\t\tpublic partial void SetLanguageVersion(global::ICSharpCode.Decompiler.CSharp.LanguageVersion languageVersion)");
 		builder.AppendLine("\t\t{");
 		builder.AppendLine("\t\t\t// By default, all decompiler features are enabled.");
 		builder.AppendLine("\t\t\t// Disable some of them based on language version:");
@@ -356,10 +356,7 @@ namespace ICSharpCode.Decompiler
 
 	static void WriteGetMinimumRequiredVersion(StringBuilder builder, IGrouping<int, SettingInfo>[] versionBuckets)
 	{
-		builder.AppendLine("\t\t/// <summary>");
-		builder.AppendLine("\t\t/// Gets the lowest language version that includes all currently enabled language features.");
-		builder.AppendLine("\t\t/// </summary>");
-		builder.AppendLine("\t\tpublic global::ICSharpCode.Decompiler.CSharp.LanguageVersion GetMinimumRequiredVersion()");
+		builder.AppendLine("\t\tpublic partial global::ICSharpCode.Decompiler.CSharp.LanguageVersion GetMinimumRequiredVersion()");
 		builder.AppendLine("\t\t{");
 		foreach (var bucket in versionBuckets.Reverse())
 		{

ICSharpCode.Decompiler/CSharp/ProjectDecompiler/WholeProjectDecompiler.cs

@@ -60,6 +60,13 @@ namespace ICSharpCode.Decompiler.CSharp.ProjectDecompiler
 
 		LanguageVersion? languageVersion;
 
+		/// <summary>
+		/// The C# language version written into the exported project file as LangVersion.
+		/// This is an export parameter, not decompiler state: when not set explicitly, it defaults
+		/// to <see cref="DecompilerSettings.GetMinimumRequiredVersion"/> of the current settings,
+		/// and an explicit value below that minimum is rejected (here and again when the export
+		/// starts) because the emitted code could not compile under it.
+		/// </summary>
 		public LanguageVersion LanguageVersion {
 			get { return languageVersion ?? Settings.GetMinimumRequiredVersion(); }
 			set {

ICSharpCode.Decompiler/DecompilerSettings.cs

@@ -44,12 +44,31 @@ namespace ICSharpCode.Decompiler
 		/// This does not imply that the resulting code strictly uses only language features from
 		/// that version. Language constructs like generics or ref locals cannot be removed from
 		/// the compiled code.
+		/// The language version is a construction shortcut, not state: it initializes the feature
+		/// flags once (see <see cref="SetLanguageVersion"/>) and is not stored afterwards.
 		/// </remarks>
 		public DecompilerSettings(CSharp.LanguageVersion languageVersion)
 		{
 			SetLanguageVersion(languageVersion);
 		}
 
+		/// <summary>
+		/// One-shot profile initializer: deactivates all language features from versions newer than
+		/// <paramref name="languageVersion"/>. The version itself is not stored - the feature flags
+		/// are the only state - so the call is not reversible and a later call with a higher version
+		/// does not re-enable features. Use <see cref="GetMinimumRequiredVersion"/> to derive a
+		/// version back from the flags.
+		/// </summary>
+		public partial void SetLanguageVersion(CSharp.LanguageVersion languageVersion);
+
+		/// <summary>
+		/// Derives the lowest language version that includes all currently enabled language
+		/// features. The settings do not store a language version, so this derivation is how a
+		/// version is recovered from the flags; project export uses it as the default (and lower
+		/// bound) for the LangVersion written into the project file.
+		/// </summary>
+		public partial CSharp.LanguageVersion GetMinimumRequiredVersion();
+
 		/// <summary>
 		/// Use C# 9 <c>nint</c>/<c>nuint</c> types.
 		/// </summary>