Add physical partiality post-refinement for stills (default on)

Replace the frozen scalar-sigma stills partiality with a physical, refined model.
Per crystal, refine an orientation tilt (dpsi_x, dpsi_y) against the running merge
and recompute each reflection's partiality analytically from the refined geometry
(angular Ewald-proximity model, sigma(d*) = gamma_e*d*), with the per-crystal scale
G profiled out by the existing robust IRLS - no re-integration. A soft Gaussian
prior on dpsi tames weak-data overfit while staying inert on strong data. The
merge <-> refine loop iterates a few times.

This is now the stills default via ScalingSettings::stills_partiality_refine (on).
A single opt-out flag `--simple-stills` reverts to treating every reflection as a
full (p=1, single pass). Retires the experimental `--still-partiality` flag. The
viewer gains a "Partiality post-refinement (stills)" checkbox in Scaling settings.

Validated (integrate-once / --scale): CC1/2 and R_meas both improve on lyso8
(+2.8 / -10), KR2 (+5.6 / -3.4) and LOV (+2.1 / -4); neutral on a pink-beam DMM
set (already-full reflections); R-free/R-work down vs a fixed model; competitive
with CrystFEL partialator on matched frames.

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::StillsPartialityRefine(bool input) {
+    stills_partiality_refine = input;
+    return *this;
+}
+
+bool ScalingSettings::GetStillsPartialityRefine() const {
+    return stills_partiality_refine;
+}
+
 ScalingSettings &ScalingSettings::ExpectedVarianceMerge(bool input) {
     expected_variance_merge = input;
     return *this;