Fix GlyphServer closing without response on oversized requests

When the receive buffer fills before headers are complete (e.g. 100KB
URL or header), send 431 instead of silently closing the connection.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: MDA2AV

src/Servers/GlyphServer/Program.cs

@@ -44,6 +44,13 @@ static async Task HandleClientAsync(TcpClient client, CancellationToken ct)
         {
             while (!ct.IsCancellationRequested)
             {
+                if (filled >= buffer.Length)
+                {
+                    // Buffer full but headers still incomplete — request is too large
+                    await stream.WriteAsync(MakeErrorResponse(431, "Request Header Fields Too Large"), ct);
+                    return;
+                }
+
                 var read = await stream.ReadAsync(buffer.AsMemory(filled), ct);
                 if (read == 0) break;
                 filled += read;