Write IL-with-C# comment slices as spans

The "IL with C#" view cut up to four substrings (prefix, trimmed
prefix, highlighted range, suffix) out of every source line emitted
alongside an IL instruction. ISmartTextOutput now accepts a
ReadOnlySpan<char> - as a default interface method falling back to
Write(text.ToString()) so existing implementers keep working - and
AvaloniaEditTextOutput appends the span straight into its
StringBuilder. The overload lives on ISmartTextOutput rather than
ITextOutput because the latter is netstandard2.0 (no default interface
methods there), where a new member would break every external
implementer of the decompiler library; the highlighted-comment path is
typed against ISmartTextOutput already.

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

ILSpy/Languages/CSharpILMixedLanguage.cs

@@ -181,13 +181,13 @@ namespace ICSharpCode.ILSpy.Languages
 				output.Write("// ");
 				output.BeginSpan(gray);
 				if (isSingleLine)
-					output.Write(text.Substring(0, startColumn).TrimStart());
+					output.Write(text.AsSpan(0, startColumn).TrimStart());
 				else
-					output.Write(text.Substring(0, startColumn));
+					output.Write(text.AsSpan(0, startColumn));
 				output.EndSpan();
-				output.Write(text.Substring(startColumn, endColumn - startColumn));
+				output.Write(text.AsSpan(startColumn, endColumn - startColumn));
 				output.BeginSpan(gray);
-				output.Write(text.Substring(endColumn));
+				output.Write(text.AsSpan(endColumn));
 				output.EndSpan();
 				output.WriteLine();
 			}

ILSpy/TextView/AvaloniaEditTextOutput.cs

@@ -186,6 +186,13 @@ namespace ICSharpCode.ILSpy.TextView
 			CheckLength();
 		}
 
+		public void Write(ReadOnlySpan<char> text)
+		{
+			WriteIndentIfNeeded();
+			builder.Append(text);
+			CheckLength();
+		}
+
 		public void WriteLine()
 		{
 			if (IgnoreNewLineAndIndent)

ILSpy/TextView/ISmartTextOutput.cs

@@ -48,6 +48,13 @@ namespace ICSharpCode.ILSpy.TextView
 		void BeginSpan(HighlightingColor highlightingColor);
 		void EndSpan();
 
+		/// <summary>
+		/// Writes a slice of text without requiring the caller to allocate an intermediate
+		/// string. Implementations that buffer internally should override the default,
+		/// which falls back to <see cref="ITextOutput.Write(string)"/>.
+		/// </summary>
+		void Write(ReadOnlySpan<char> text) => Write(text.ToString());
+
 		/// <summary>
 		/// Title displayed in the document tab's header.
 		/// </summary>