jfjoch_process: optional absorption surface for rot3d scaling (--absorption)

Adds an opt-in smooth absorption correction for rotation scaling. After the
rot3d fulls are scaled, --absorption[=num] fits a multiplicative surface
A(s1_crystal) - a degree<=4 monomial basis (real spherical harmonics up to l=4,
as XDS/DIALS) of the diffracted-beam direction in the crystal/goniometer frame,
by ridge-regularized log-linear least-squares of I_scaled/I_ref weighted by
(I/sigma)^2, over num iterations (default 3); the surface divides image_scale_corr
and the fulls are re-merged.

Off by default and a no-op without rot3d. On the test panel (~13 keV, thin
crystals) it is metric-neutral - fitted rms(log A) ~3-4%, ISa/CC1/2 unchanged -
because absorption is negligible there and the per-frame scale G(phi) already
absorbs the angular part. It is kept as a lever for low-energy data (e.g. 6 keV)
where absorption becomes significant. Stored as ScalingSettings::absorption_iter.

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

common/ScalingSettings.cpp

@@ -149,6 +149,17 @@ bool ScalingSettings::GetScaleFulls() const {
     return scale_fulls;
 }
 
+ScalingSettings &ScalingSettings::AbsorptionIter(int input) {
+    if (input < 0)
+        throw JFJochException(JFJochExceptionCategory::InputParameterInvalid, "Absorption iterations must be non-negative");
+    absorption_iter = input;
+    return *this;
+}
+
+int ScalingSettings::GetAbsorptionIter() const {
+    return absorption_iter;
+}
+
 ScalingSettings &ScalingSettings::SmoothGWindow(int input) {
     if (input < 0)
         throw JFJochException(JFJochExceptionCategory::InputParameterInvalid, "Smooth-G window must be non-negative");