rugnux: veto a b-ballooning merohedral-twin over-promotion (space-group search)

The systematic-b test (a genuine symmetry step barely moves the merge error
model's b; a merohedral twin, forcing non-equivalent reflections together,
balloons it) was only ever a RESCUE - it could promote a chi^2-borderline
genuine step but never demote a chi^2-passing one. So a partial twin whose
within-orbit scatter looks self-consistent (merged chi^2 below the ratio
bound) slipped through on chi^2 alone and was over-promoted to the holohedral
group, ballooning b and collapsing the merged ISa.

Make the same test a VETO: a chi^2-passing high-symmetry promotion whose b,
relative to the largest confirmed subgroup, exceeds max_systematic_b_veto is
kept in its true lower symmetry. Calibrated on the rotation-test battery,
where the largest genuine step (a P422 tetragonal) sits at b-ratio ~1.8 and a
merohedral R3->R32 twin at ~2.6 - an empty gap - so 2.0 catches the twin while
never demoting a genuine high-symmetry merge.

Battery space-group match 20->21/25 (an R3 case previously merged as R32 is
now correct), every other crystal unchanged.

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

image_analysis/scale_merge/SearchSpaceGroup.cpp

@@ -465,14 +465,14 @@ SearchSpaceGroupResult SearchSpaceGroup(
         bool consistent = c.pg->rotations.empty() || !std::isfinite(c.chi2) ||
                           !std::isfinite(chi2_ref) || c.chi2 <= chi2_ref * opt.max_merge_chi2_ratio;
 
-        // Rescue a genuine high-symmetry merge whose chi^2 lands just past the ratio bound because its
-        // data are imperfectly scaled (see max_merge_chi2_rescue). The systematic-error test tells it
-        // apart from a twin: promote only if the extra intensity-proportional error b, relative to the
-        // largest confirmed subgroup (by rotation-set inclusion), stayed within max_systematic_b_ratio -
-        // a genuine step barely moves it, a twin's balloons.
-        if (!consistent && std::isfinite(c.chi2) && std::isfinite(chi2_ref)
-            && c.chi2 <= chi2_ref * opt.max_merge_chi2_rescue) {
-            double parent_b = -1.0;
+        // Systematic-error test vs the largest confirmed subgroup (by rotation-set inclusion): merging
+        // under a genuine operator gains multiplicity without intensity-proportional disagreement, so the
+        // merge error model's b barely moves; a merohedral twin forces non-equivalent reflections together
+        // and b balloons. It both RESCUES a genuine step whose chi^2 drifts just past the ratio bound
+        // (imperfectly scaled data) and VETOES a twin whose chi^2 now looks self-consistent but whose b
+        // balloons - the chi^2 ratio alone no longer separates them.
+        double parent_b = -1.0;
+        if (!c.pg->rotations.empty()) {
             int parent_order = 0;
             for (const auto& s : pg_cands)
                 if (s.order < c.order && s.order > parent_order
@@ -481,9 +481,16 @@ SearchSpaceGroupResult SearchSpaceGroup(
                     parent_order = s.order;
                     parent_b = s.b_extra;
                 }
-            if (parent_b > 1e-4 && c.b_extra <= parent_b * opt.max_systematic_b_ratio)
-                consistent = true;
         }
+        if (!consistent && std::isfinite(c.chi2) && std::isfinite(chi2_ref)
+            && c.chi2 <= chi2_ref * opt.max_merge_chi2_rescue
+            && parent_b > 1e-4 && c.b_extra <= parent_b * opt.max_systematic_b_ratio)
+            consistent = true;
+        // Veto a chi^2-passing promotion whose b clearly ballooned (above the largest genuine step, below a
+        // twin); a genuine but imperfectly-scaled high-symmetry merge stays under the bound and is untouched.
+        if (consistent && parent_b > 1e-4 && c.b_extra > parent_b * opt.max_systematic_b_veto)
+            consistent = false;
+
         if (!consistent)
             continue;
         if (c.order > best_pg_order || (c.order == best_pg_order && c.min_class_cc > best_pg_min_cc)) {

image_analysis/scale_merge/SearchSpaceGroup.h

@@ -106,6 +106,17 @@ struct SearchSpaceGroupOptions {
     double max_merge_chi2_rescue = 2.30;
     double max_systematic_b_ratio = 1.90;
 
+    // Veto bound. The systematic-b test above is otherwise only a rescue - it can promote a chi^2-borderline
+    // genuine step but never demote a chi^2-passing one. A merohedral twin whose within-orbit scatter looks
+    // self-consistent (chi^2 ratio below max_merge_chi2_ratio) therefore slips through on chi^2 alone even
+    // though merging its non-equivalent reflections balloons b. So a chi^2-passing high-symmetry promotion
+    // whose b, relative to the largest confirmed subgroup, exceeds this bound is vetoed and kept in its true
+    // lower symmetry. Calibrated on the rotation-test battery, where the largest genuine step (a P422
+    // tetragonal) sits at b-ratio ~1.8 and a merohedral R3->R32 twin at ~2.6 - an empty gap - so 2.0 keeps
+    // every genuine high-symmetry merge (they are never demoted) while catching the twin. The veto only ever
+    // keeps a clearly-ballooned promotion down, never promotes.
+    double max_systematic_b_veto = 2.0;
+
     // --- Stage B: space group (screw axes / centering) ---
     bool determine_space_group = true;     // false: stop at the symmorphic representative