Utf8StringWriter<T>.AppendLine shadows the eponymous extension method.

[markk-df]

The Utf8StringWriter<T> type has:

• Instance method: void AppendLine(string s)
• Extension method: void AppendLine(ref Utf8StringWriter<T>, [InterpolatedStringHandler] ref Utf8StringWriter<T>)

Due to C# overload resolution rules, calling writer.AppendLine($"...") always resolves to the instance method, causing the interpolated string to be evaluated via string.Format (allocations + boxing) instead of using the zero-allocation handler.

This defeats the primary purpose of the library and creates a silent performance trap.