stills geometry refinement: a distance given with --detector-distance is held in the joint fit

The joint distance+cell fit freed the distance even where the user had given it. A user-given distance
is a hard pin: the cell alone absorbs any error. The file's distance and DecideStillsGeometry's stay
free as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015CeEGgpKdfkCTJb3YNMJcu

rugnux/RugnuxStills.cpp

@@ -282,9 +282,9 @@ void Rugnux::RefineStillsGeometry(int start_image, int end_image, int images_to_
     // short, with the cell absorbing it, mispredicted the spots past 2.5 A by enough to make the merged
     // intensities there negative. A deposited cell is often a percent off this crystal (another
     // temperature, or the cell the depositors started from), and a stated distance can be off as much,
-    // so the cell (-C) and the distance (the file's, --detector-distance or DecideStillsGeometry's) are
-    // both only where the fit starts.
-    gr_settings.refine_distance = true;
+    // so the cell (-C) and the file's distance (or DecideStillsGeometry's) are both only where the fit
+    // starts. A distance the user gave (--detector-distance) is held: the cell alone absorbs any error.
+    gr_settings.refine_distance = !config_.detector_distance_given;
     gr_settings.cell_reg_coeff = 0.0;
 
     const DiffractionGeometry nominal = experiment_.GetDiffractionGeometry();