Remove the global (joint) scaling path

Profiling showed the joint Ceres solve took ~120 s versus ~3.5 s for the
alternating per-image scaling loop (~35x) for no quality gain (HEWL
anomalous 0.54x vs 0.53x), so it is not worth keeping. Drop
GlobalScale.{h,cpp}, the jfjoch_process --global-scale flag, and
ScalingSettings::GlobalScaling.

While here, in the same scaling/process area: fold scale_fulls into
ScalingSettings (alongside combine_3d) so the CLI and experiment carry it
uniformly, add per-substep [timing] logging to the scaling/merge post-pass
(including the serial MergeAll vs parallel ScaleAllImages split), and carry
structured MergeStatistics + ISa in ProcessResult.

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

common/ScalingSettings.cpp

@@ -133,6 +133,15 @@ bool ScalingSettings::GetCombine3D() const {
     return combine_3d;
 }
 
+ScalingSettings &ScalingSettings::ScaleFulls(bool input) {
+    scale_fulls = input;
+    return *this;
+}
+
+bool ScalingSettings::GetScaleFulls() const {
+    return scale_fulls;
+}
+
 double ScalingSettings::GetMinPartiality() const {
     return min_partiality;
 }