Debias stills merge with expected-variance weighting

The serial-stills merge (MergeOnTheFly::CorrectedSigma) weighted each
observation by 1/sigma^2 using the observation's OWN sigma. Below ~1
photon the Poisson signal part of that sigma correlates with the
observation's up/down fluctuation, so the inverse-variance mean is
biased low: an up-fluctuated observation acquires a larger sigma and is
over-downweighted. The rotation combine (RotationScaleMerge::
process_rawrun) already avoids this by rebuilding the signal variance at
the pooled estimate; the stills path did not.

Decompose each observation's variance into a background/read part (kept
per-observation) and a Poisson signal part, and rebuild the signal part
at the reflection's expected <I>. Bit-identical when an observation sits
at its reflection mean; only weak-shell weights move. Now default on, so
the stills path matches the rotation path;
--no-expected-variance-merge restores the old observed-sigma weighting.

Validated by paired refinement (phenix, 5 free-set seeds, byte-identical
free flags across arms): R-free-neutral on strong lysozyme and lower
R-free on weak serial-stills data checked against an independent
deposited model (6/6 seeds). The CC1/2 dip on strong data reflects
precision, not accuracy. Applies to both offline rugnux and the online
broker stills merge.

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

common/ScalingSettings.cpp

@@ -160,6 +160,15 @@ bool ScalingSettings::GetStillsModulation() const {
     return stills_modulation;
 }
 
+ScalingSettings &ScalingSettings::ExpectedVarianceMerge(bool input) {
+    expected_variance_merge = input;
+    return *this;
+}
+
+bool ScalingSettings::GetExpectedVarianceMerge() const {
+    return expected_variance_merge;
+}
+
 ScalingSettings &ScalingSettings::SmoothGDegrees(double input) {
     if (input < 0)
         throw JFJochException(JFJochExceptionCategory::InputParameterInvalid, "Smooth-G range must be non-negative");