rugnux: header line for the per-image _image.dat scaling table

The offline per-image scaling table had bare numeric columns. Prefix it with
a '#' comment header (image_number scale_G bfactor_Ang2 mosaicity_deg
wedge_deg cc_to_merge cc_n) so it is self-describing; numpy.loadtxt / gnuplot
skip the comment line, so existing consumers are unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

image_analysis/scale_merge/ScalingResult.cpp

@@ -40,6 +40,9 @@ void ScalingResult::SaveToFile(const std::string &filename) {
                               "Cannot open " + img_path + " for writing");
     }
 
+    // Header so the columns are self-describing (lines starting with '#' are comments for gnuplot/numpy).
+    img_file << "# image_number scale_G bfactor_Ang2 mosaicity_deg wedge_deg cc_to_merge cc_n\n";
+
     for (size_t i = 0; i < image_scale_g.size(); ++i) {
         img_file << i
                 << " " << image_scale_g[i]