Merge: make outlier rejection a ScalingSettings parameter + CLI flag, default 6

Promote the per-observation merge outlier rejection from the temporary PR_REJECT
env knob to a real setting: ScalingSettings::OutlierRejectNsigma (default 6, <=0
disables), driven into MergeOnTheFly via the constructor, with a --reject-outliers
CLI flag in jfjoch_process. Default-on at 6 sigma matches XDS (MISFITS/REJECT) and
DIALS (normalised-deviation test); it validated on the full-jet R-free (0.2625 ->
0.2585). Applies to both the PixelRefine and classical merge paths.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

common/ScalingSettings.cpp

@@ -115,6 +115,15 @@ ScalingSettings &ScalingSettings::MinCCForImage(double input) {
     return *this;
 }
 
+double ScalingSettings::GetOutlierRejectNsigma() const {
+    return outlier_reject_nsigma;
+}
+
+ScalingSettings &ScalingSettings::OutlierRejectNsigma(double input) {
+    outlier_reject_nsigma = input; // <= 0 disables; no upper bound (large = effectively off)
+    return *this;
+}
+
 double ScalingSettings::GetMinPartiality() const {
     return min_partiality;
 }