Stills partiality: an unmeasurable CC is not a reason to adopt the refined tilt

The "keep what the crystal came in with" gate required std::isfinite(cc) before it
would reject, so a refined model whose CC could not be measured at all was adopted.
ImageReferenceCC returns NaN when fewer than 20 reflections clear the partiality
cut - which is exactly what a refinement that collapsed the partialities produces,
since the cut is on the partialities it just rewrote. The gate therefore failed
open on precisely the crystals it exists to catch, and wrote the NaN into
image_scale_cc, on which --min-image-cc then drops the image from the merge, the
error model and the statistics.

Treat a CC that cannot be measured as worse than one that can, so the crystal is
put back exactly as it arrived.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

image_analysis/scale_merge/StillsPartialityRefine.cpp

@@ -326,8 +326,11 @@ double StillsPartialityRefine::RefineOne(IntegrationOutcome &outcome,
 
     // Adopt the refined model only if it correlates with the reference at least as well as the model it
     // replaces. Rejecting puts the crystal back exactly as it arrived, which is the same state a
-    // crystal with too few reflections to fit ends in.
-    if (cc_before.has_value() && std::isfinite(*cc_before) && std::isfinite(cc) && cc < *cc_before) {
+    // crystal with too few reflections to fit ends in. A CC that cannot be measured at all counts as
+    // worse, not as no opinion: ImageReferenceCC returns NaN when too few reflections clear the
+    // partiality cut, which is precisely what a refinement that collapsed the partialities produces -
+    // and adopting it would leave image_scale_cc NaN, on which --min-image-cc drops the image outright.
+    if (cc_before.has_value() && std::isfinite(*cc_before) && !(std::isfinite(cc) && cc >= *cc_before)) {
         for (size_t i = 0; i < outcome.reflections.size(); ++i) {
             outcome.reflections[i].partiality = before[i].first;
             outcome.reflections[i].image_scale_corr = before[i].second;