RotationScaleMerge: a merge with no rocking-curve runs reports itself instead of dividing by zero

With more than one thread, Combine() split its runs into per-thread chunks of
ceil(n_run / nt) with nt = min(nthreads, n_run); with no runs at all (a lattice
that integrated nothing) nt was 0 and the process died on SIGFPE in Step 5.
The empty case now takes the serial path, and the merge ends in the ordinary
exception. Test: RotationScaleMerge_NoReflectionsIsAnErrorNotACrash (fails
without the fix, on the host path).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HVeDQqN9qRD57vnShzTC37

image_analysis/scale_merge/RotationScaleMerge.cpp

@@ -4843,7 +4843,9 @@ void RotationScaleMerge::Combine() {
     size_t n_used = 0;
     std::vector<std::vector<Obs>> part;   // the fulls of consecutive runs, in run order
 
-    if (observation_dump || nthreads <= 1) {
+    // No runs at all (a lattice that integrated nothing) takes the serial path too: the chunking below
+    // divides by the number of threads it would use, which is then 0.
+    if (observation_dump || nthreads <= 1 || n_run == 0) {
         // Serial (diagnostic dump needs a single writer).
         std::ostringstream dump;
         if (observation_dump) {

tests/RotationScaleWalkTest.cpp

@@ -281,3 +281,20 @@ TEST_CASE("TrimBlankSweepEnds_WeakCrystalKeepsItsEnds", "[RotationScale]") {
     CHECK(sweep.front().reflections.size() == 200);
     CHECK(sweep.back().reflections.size() == 200);
 }
+
+// A lattice that integrated nothing hands the merge frames without a single reflection. On more than one
+// thread the 3D combine split its rocking-curve runs into per-thread chunks by dividing by the number of
+// threads it would use - 0 with no runs - and the process died on SIGFPE instead of reporting the empty merge.
+TEST_CASE("RotationScaleMerge_NoReflectionsIsAnErrorNotACrash", "[RotationScale]") {
+    DiffractionExperiment x(DetJF(1));
+    x.Goniometer(GoniometerAxis("omega", 0.0f, 0.2f, Coord(1, 0, 0), std::nullopt));
+    x.BeamX_pxl(512).BeamY_pxl(256).DetectorDistance_mm(150).IncidentEnergy_keV(WVL_1A_IN_KEV);
+    x.SetUnitCell(UnitCell(50, 60, 70, 90, 90, 90));
+    std::vector<IntegrationOutcome> outcomes(20);
+    for (auto &o : outcomes)
+        o.geom = x.GetDiffractionGeometry();
+    Logger logger("RotationScaleMerge_NoReflections");
+    RotationScaleMerge rsm(x, outcomes, x.GetUnitCell(), 3, 4, logger);
+    rsm.Ingest();
+    REQUIRE_THROWS_AS(rsm.Run(false, true, false), JFJochException);
+}